Structures, Unions, Enums - General Questions

What will be the output of the program?

#include <stdio.h>
union U { int i; char c; };
int main(void) {
    printf("%zu\n", sizeof(union U));
    return 0;
}

What will be the output of the program?

#include <stdio.h>
struct P { int a; int b; };
int main(void) {
    struct P p = {1, 2}, q;
    q = p;
    q.a = 9;
    printf("%d %d\n", p.a, q.a);
    return 0;
}

Which keyword creates a structure type in C?

What will be the output of the program?

#include <stdio.h>
struct S { int x; };
void set(struct S *s) { s->x = 100; }
int main(void) {
    struct S v = {5};
    set(&v);
    printf("%d\n", v.x);
    return 0;
}

What is the key difference between a structure and a union?