diff --git a/【额外练习】选择结构/7-10 选择-三角形分类.c b/【额外练习】选择结构/7-10 选择-三角形分类.c
new file mode 100644
index 0000000..b6856c7
--- /dev/null
+++ b/【额外练习】选择结构/7-10 选择-三角形分类.c	
@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int main() {
+    int a, b, c;
+
+    scanf("%d%d%d", &a, &b, &c);
+
+    if (a == b && b == c) {
+        printf("bian\n");
+    } else if (a == b || b == c || a == c) {
+        printf("yao\n");
+    } else {
+        printf("triangle\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-11 选择-五分制转百分制.c b/【额外练习】选择结构/7-11 选择-五分制转百分制.c
new file mode 100644
index 0000000..07f488f
--- /dev/null
+++ b/【额外练习】选择结构/7-11 选择-五分制转百分制.c	
@@ -0,0 +1,21 @@
+#include <stdio.h>
+
+int main() {
+    char grade;
+
+    scanf("%c", &grade);
+
+    if (grade == 'A') {
+        printf("85~100\n");
+    } else if (grade == 'B') {
+        printf("70~84\n");
+    } else if (grade == 'C') {
+        printf("60~69\n");
+    } else if (grade == 'D') {
+        printf("<60\n");
+    } else {
+        printf("Error\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-12 选择-大小写转换.c b/【额外练习】选择结构/7-12 选择-大小写转换.c
new file mode 100644
index 0000000..9e69797
--- /dev/null
+++ b/【额外练习】选择结构/7-12 选择-大小写转换.c	
@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+int main() {
+    char c;
+
+    scanf("%c", &c);
+
+    if (c >= 'a' && c <= 'z') {
+        printf("%c\n", c - 32);
+    } else if (c >= 'A' && c <= 'Z') {
+        printf("%c\n", c + 32);
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-13 选择-体积最大.c b/【额外练习】选择结构/7-13 选择-体积最大.c
new file mode 100644
index 0000000..8726088
--- /dev/null
+++ b/【额外练习】选择结构/7-13 选择-体积最大.c	
@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int main() {
+    int a, b, c;
+
+    scanf("%d%d%d", &a, &b, &c);
+
+    if (a > b && a > c) {
+        printf("%d\n", a);
+    } else if (b > a && b > c) {
+        printf("%d\n", b);
+    } else {
+        printf("%d\n", c);
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-14 选择-N以内所有正整数和.c b/【额外练习】选择结构/7-14 选择-N以内所有正整数和.c
new file mode 100644
index 0000000..0e7d315
--- /dev/null
+++ b/【额外练习】选择结构/7-14 选择-N以内所有正整数和.c	
@@ -0,0 +1,16 @@
+#include <stdio.h>
+
+int main() {
+    int n;
+    int sum = 0;
+
+    scanf("%d", &n);
+
+    for (int i = 1; i <= n; i++) {
+        sum += i;
+    }
+
+    printf("%d\n", sum);
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-15 选择-简易计算机.c b/【额外练习】选择结构/7-15 选择-简易计算机.c
new file mode 100644
index 0000000..23eca21
--- /dev/null
+++ b/【额外练习】选择结构/7-15 选择-简易计算机.c	
@@ -0,0 +1,51 @@
+#include <stdio.h>
+
+int main() {
+    int x, a, b, c;
+
+    scanf("%d%d%d%d", &x, &a, &b, &c);
+
+    switch (x) {
+        case 0:
+            printf("%d", a + b);
+            break;
+
+        case 1:
+            printf("%d", a - b);
+            break;
+
+        case 2:
+            printf("%d", a * b);
+            break;
+
+        case 3:
+            printf("%d", a / b);
+            break;
+
+        case 4:
+            printf("%d", a % b);
+            break;
+
+        case 5:
+            printf("%d", a ? b : c);
+            break;
+
+        case 6:
+            printf("%d", a > b);
+            break;
+
+        case 7:
+            printf("%d", a < b);
+            break;
+
+        case 8:
+            printf("%d", a != b);
+            break;
+
+        case 9:
+            printf("%d", a == b);
+            break;
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-16 选择-简单的运算.c b/【额外练习】选择结构/7-16 选择-简单的运算.c
new file mode 100644
index 0000000..d056bd9
--- /dev/null
+++ b/【额外练习】选择结构/7-16 选择-简单的运算.c	
@@ -0,0 +1,12 @@
+#include <math.h>
+#include <stdio.h>
+
+int main() {
+    double n;
+
+    scanf("%lf", &n);
+
+    printf("%d %d %.5lf\n", (int)ceil(n), (int)floor(n), n);
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-17 选择-水仙花数.c b/【额外练习】选择结构/7-17 选择-水仙花数.c
new file mode 100644
index 0000000..54682b6
--- /dev/null
+++ b/【额外练习】选择结构/7-17 选择-水仙花数.c	
@@ -0,0 +1,19 @@
+#include <stdio.h>
+
+int main() {
+    int n;
+
+    scanf("%d", &n);
+
+    int a = n / 100;
+    int b = n / 10 % 10;
+    int c = n % 10;
+
+    if (n == a * a * a + b * b * b + c * c * c) {
+        printf("yes\n");
+    } else {
+        printf("no\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-18 选择-三角形面积.c b/【额外练习】选择结构/7-18 选择-三角形面积.c
new file mode 100644
index 0000000..f149db7
--- /dev/null
+++ b/【额外练习】选择结构/7-18 选择-三角形面积.c	
@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int main() {
+    double a, b, c;
+
+    scanf("%lf%lf%lf", &a, &b, &c);
+
+    if (a + b > c && a + c > b && b + c > a) {
+        double p = (a + b + c) / 2;
+        double s = sqrt(p * (p - a) * (p - b) * (p - c));
+        printf("%.2lf %.2lf\n", s, a + b + c);
+    } else {
+        printf("no triangle\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-19 选择-完全平方数.c b/【额外练习】选择结构/7-19 选择-完全平方数.c
new file mode 100644
index 0000000..18e7cc7
--- /dev/null
+++ b/【额外练习】选择结构/7-19 选择-完全平方数.c	
@@ -0,0 +1,18 @@
+#include <math.h>
+#include <stdio.h>
+
+int main() {
+    int n;
+
+    scanf("%d", &n);
+
+    int m = sqrt(n);
+
+    if (m * m == n) {
+        printf("Yes\n");
+    } else {
+        printf("No\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-20 选择-矩形与点的位置.c b/【额外练习】选择结构/7-20 选择-矩形与点的位置.c
new file mode 100644
index 0000000..8084886
--- /dev/null
+++ b/【额外练习】选择结构/7-20 选择-矩形与点的位置.c	
@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+int main() {
+    int xL, yL, xR, yR, x, y;
+
+    scanf("%d%d%d%d%d%d", &xL, &yL, &xR, &yR, &x, &y);
+
+    if (x >= xL && x <= xR && y <= yL && y >= yR) {
+        printf("Yes\n");
+    } else {
+        printf("No\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-21 选择-数的判别.c b/【额外练习】选择结构/7-21 选择-数的判别.c
new file mode 100644
index 0000000..62538f6
--- /dev/null
+++ b/【额外练习】选择结构/7-21 选择-数的判别.c	
@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int main() {
+    int n;
+
+    scanf("%d", &n);
+
+    if (n % 2 == 0) {
+        printf("%d is an even number\n", n);
+    } else if (n % 3 == 0) {
+        printf("%d is an odd number,it can be divided by 3\n", n);
+    } else {
+        printf("%d is an odd number,it can't be divided by 3\n", n);
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-22 选择-销售菜单.c b/【额外练习】选择结构/7-22 选择-销售菜单.c
new file mode 100644
index 0000000..1d86626
--- /dev/null
+++ b/【额外练习】选择结构/7-22 选择-销售菜单.c	
@@ -0,0 +1,31 @@
+#include <stdio.h>
+
+int main() {
+    int n;
+
+    scanf("%d", &n);
+
+    switch (n) {
+        case 1:
+            printf("8\n");
+            break;
+
+        case 2:
+            printf("4.5\n");
+            break;
+
+        case 3:
+            printf("12.8\n");
+            break;
+
+        case 4:
+            printf("3\n");
+            break;
+
+        default:
+            printf("Exit\n");
+            break;
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-23 选择-电话费.c b/【额外练习】选择结构/7-23 选择-电话费.c
new file mode 100644
index 0000000..0f83d02
--- /dev/null
+++ b/【额外练习】选择结构/7-23 选择-电话费.c	
@@ -0,0 +1,20 @@
+#include <stdio.h>
+
+int main() {
+    int t;
+
+    scanf("%d", &t);
+
+    double s1 = 25 + 0.1 * t;
+    double s2 = 20;
+
+    if (t >= 100) s2 = 0.3 * t;
+
+    if (s1 > s2) {
+        printf("2\n");
+    } else {
+        printf("1\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-24 选择-税收计算1.c b/【额外练习】选择结构/7-24 选择-税收计算1.c
new file mode 100644
index 0000000..18c30fd
--- /dev/null
+++ b/【额外练习】选择结构/7-24 选择-税收计算1.c	
@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int main() {
+    int x;
+
+    scanf("%d", &x);
+
+    if (x <= 800) {
+        printf("0.0\n");
+    } else if (x <= 4000) {
+        printf("%.1lf\n", (x - 800) * 0.15);
+    } else {
+        printf("%.1lf\n", x * 0.1);
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-25 选择-自然数.c b/【额外练习】选择结构/7-25 选择-自然数.c
new file mode 100644
index 0000000..0f3597e
--- /dev/null
+++ b/【额外练习】选择结构/7-25 选择-自然数.c	
@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+int main() {
+    double a;
+
+    scanf("%lf", &a);
+
+    if (a >= 0 && (int)a == a) {
+        printf("Yes\n");
+    } else {
+        printf("No\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-26 选择-宝藏.c b/【额外练习】选择结构/7-26 选择-宝藏.c
new file mode 100644
index 0000000..4baf3f1
--- /dev/null
+++ b/【额外练习】选择结构/7-26 选择-宝藏.c	
@@ -0,0 +1,31 @@
+#include <stdio.h>
+
+int main() {
+    int x;
+
+    scanf("%d", &x);
+
+    int canFill = 0;
+
+    for (int a = 0; a <= x / 4; a++) {
+        for (int b = 0; b <= x / 5; b++) {
+            for (int c = 0; c <= x / 7; c++) {
+                // 计算当前组合的体积
+                if (4 * a + 5 * b + 7 * c == x) {
+                    canFill = 1;
+                    break;
+                }
+            }
+            if (canFill) break;
+        }
+        if (canFill) break;
+    }
+
+    if (canFill) {
+        printf("ni bu yao guo lai a!\n");
+    } else {
+        printf("What a lucky day!\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-27 选择-回文数.c b/【额外练习】选择结构/7-27 选择-回文数.c
new file mode 100644
index 0000000..b8bbfaf
--- /dev/null
+++ b/【额外练习】选择结构/7-27 选择-回文数.c	
@@ -0,0 +1,24 @@
+#include <stdio.h>
+
+int main() {
+    int n;
+    int a, b, c, d, e, f, g;
+
+    scanf("%d", &n);
+
+    a = n / 1000000;
+    b = n / 100000 % 10;
+    c = n / 10000 % 10;
+    d = n / 1000 % 10;
+    e = n / 100 % 10;
+    f = n / 10 % 10;
+    g = n % 10;
+
+    if (a == g && b == f && c == e) {
+        printf("Yes\n");
+    } else {
+        printf("No\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-28 选择-时钟.c b/【额外练习】选择结构/7-28 选择-时钟.c
new file mode 100644
index 0000000..39f7fd8
--- /dev/null
+++ b/【额外练习】选择结构/7-28 选择-时钟.c	
@@ -0,0 +1,16 @@
+#include <stdio.h>
+
+int main() {
+    int h, m;
+
+    scanf("%d:%d", &h, &m);
+
+    double angle = 30 * h - 5.5 * m;
+
+    if (angle < 0) angle = -angle;
+    if (angle > 180) angle = 360 - angle;
+
+    printf("%.3lf\n", angle);
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-29 选择-求解一元二次方程.c b/【额外练习】选择结构/7-29 选择-求解一元二次方程.c
new file mode 100644
index 0000000..127613b
--- /dev/null
+++ b/【额外练习】选择结构/7-29 选择-求解一元二次方程.c	
@@ -0,0 +1,31 @@
+#include <math.h>
+#include <stdio.h>
+
+int main() {
+    double a, b, c;
+
+    scanf("%lf%lf%lf", &a, &b, &c);
+
+    double delta = b * b - 4 * a * c;
+
+    if (delta > 0) {
+        double x1 = (-b - sqrt(delta)) / (a * 2);
+        double x2 = (-b + sqrt(delta)) / (a * 2);
+
+        if (x1 > x2) {
+            double temp = x1;
+            x1 = x2;
+            x2 = temp;
+        }
+
+        printf("%.2lf %.2lf\n", x1, x2);
+    } else if (delta == 0) {
+        double x1 = -b / (a * 2);
+
+        printf("%.2lf\n", x1);
+    } else {
+        printf("No solution\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-30 选择-硬币游戏.c b/【额外练习】选择结构/7-30 选择-硬币游戏.c
new file mode 100644
index 0000000..7e0cd67
--- /dev/null
+++ b/【额外练习】选择结构/7-30 选择-硬币游戏.c	
@@ -0,0 +1,20 @@
+#include <stdio.h>
+
+int main() {
+    int a, b;
+
+    scanf("%d%d", &a, &b);
+
+    // [Theorem]
+    // In a normal nim game, the player making the first move has a winning strategy
+    // if and only if the nim-sum of the sizes of the heaps is not zero. Otherwise,
+    // the second player has a winning strategy.
+    // Source: https://en.wikipedia.org/wiki/Nim
+    if ((a ^ b) == 0) {
+        printf("abcdxyzk\n");
+    } else {
+        printf("AekdyCoin\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-31 选择-猜数字游戏.c b/【额外练习】选择结构/7-31 选择-猜数字游戏.c
new file mode 100644
index 0000000..0f9d973
--- /dev/null
+++ b/【额外练习】选择结构/7-31 选择-猜数字游戏.c	
@@ -0,0 +1,29 @@
+#include <stdio.h>
+
+int main() {
+    int total, andy, ban;
+
+    scanf("%d%d%d", &total, &andy, &ban);
+
+    int andyDiff = total - andy;
+    int banDiff = total - ban;
+
+    if (andyDiff < 0) andyDiff = -andyDiff;
+    if (banDiff < 0) banDiff = -banDiff;
+
+    if (andyDiff == banDiff) {
+        if (andy < ban) {
+            printf("Andy\n");
+        } else if (andy == ban) {
+            printf("Peace\n");
+        } else {
+            printf("Ban\n");
+        }
+    } else if (andyDiff < banDiff) {
+        printf("Andy\n");
+    } else {
+        printf("Ban\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-32 选择-扑克.c b/【额外练习】选择结构/7-32 选择-扑克.c
new file mode 100644
index 0000000..c618515
--- /dev/null
+++ b/【额外练习】选择结构/7-32 选择-扑克.c	
@@ -0,0 +1,21 @@
+#include <stdio.h>
+
+int main() {
+    int n;
+
+    scanf("%d", &n);
+
+    if (n >= 1 && n <= 10) {
+        printf("%d\n", n);
+    } else if (n == 11) {
+        printf("J\n");
+    } else if (n == 12) {
+        printf("Q\n");
+    } else if (n == 13) {
+        printf("K\n");
+    } else {
+        printf("NO\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-8 选择-买奶茶.c b/【额外练习】选择结构/7-8 选择-买奶茶.c
new file mode 100644
index 0000000..e707c21
--- /dev/null
+++ b/【额外练习】选择结构/7-8 选择-买奶茶.c	
@@ -0,0 +1,19 @@
+#include <stdio.h>
+
+int main() {
+    int x;
+
+    scanf("%d", &x);
+
+    if (x == 5) {
+        printf("6\n");
+    } else if (x == 6) {
+        printf("10\n");
+    } else if (x == 7) {
+        printf("8\n");
+    } else {
+        printf("4\n");
+    }
+
+    return 0;
+}
diff --git a/【额外练习】选择结构/7-9 选择-三角形判别.c b/【额外练习】选择结构/7-9 选择-三角形判别.c
new file mode 100644
index 0000000..347c6a8
--- /dev/null
+++ b/【额外练习】选择结构/7-9 选择-三角形判别.c	
@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+int main() {
+    int a, b, c;
+
+    scanf("%d%d%d", &a, &b, &c);
+
+    if (a + b > c && a + c > b && b + c > a) {
+        printf("Yes\n");
+    } else {
+        printf("No\n");
+    }
+
+    return 0;
+}