C中的语法错误

int a = 10;

int a = 10

int main() {
    printf("Hello, World!");
}

int main() {
    int a = 10;
    int b = 20;
    int c = a + b;
    printf("%d", c);
}

int main() {
    int a = 10;
    int b = 20;
    int c = add(a, b);
    printf("%d", c);
}

int add(int x, int y) {
    return x + y;
}

int main() {
    int a = 10;
    float b = 20.5;
    int c = a + b; // 错误：不能将int类型和float类型的变量直接相加
    printf("%d", c);
}

int main() {
    int a = 10;
    int a = 20; // 错误：不能重复定义变量a
    printf("%d", a);
}

int main() {
    int a = 10;
    /*
    int b = 20;
    printf("%d", b);
}