Structures, Unions, Enums - General Questions

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

What is the size of a union?

What will be the output of the program?

#include <stdio.h>
struct N { int v; struct N *next; };
int main(void) {
    struct N c = {3, NULL}, b = {2, &c}, a = {1, &b};
    int s = 0;
    struct N *p;
    for (p = &a; p; p = p->next) s += p->v;
    printf("%d\n", s);
    return 0;
}

What will be the output of the program?

#include <stdio.h>
struct Date { int d, m, y; };
int main(void) {
    struct Date t = {.y = 2024, .d = 15};
    printf("%d %d %d\n", t.d, t.m, t.y);
    return 0;
}

Can a structure contain a pointer to its own type?