In which header file is the NULL macro defined?

In which header file is the NULL macro defined?

2) What will be the output of the following C code?

 #include <stdio.h>
 int x = 0;
 void main()
 {
     int *ptr = &x;
     printf("%p\n", ptr);
     x++;
     printf("%p\n ", ptr);
 }

3) What will be the output of the following C code?

#include <stdio.h>
 int main()
 {
    char *p = NULL;
    char *q = 0;
    if (p)
 	  printf(" p ");
    else
       printf("nullp");
    if (q)
       printf("q\n");
    else
       printf(" nullq\n");
 }

4) What will be the output of the program ?

 #include<stdio.h>
 int main()
 {
     static char *s[] = {"black", "white", "pink", "violet"};
     char **ptr[] = {s+3, s+2, s+1, s}, ***p;
     p = ptr;
     ++p;
     printf("%s", **p+1);
     return 0;
 }

5) Which is an indirection operator among the following?