C Preprocessor - General Questions

What will be the output of the program?

#include <stdio.h>
#define STR(x) #x
int main(void) {
    printf("%s\n", STR(hello));
    return 0;
}

What will be the output of the program?

#include <stdio.h>
#define MAX(a, b) ((a) > (b) ? (a) : (b))
int main(void) {
    printf("%d\n", MAX(4, 9));
    return 0;
}

What will be the output of the program?

#include <stdio.h>
#define SQUARE(x) x * x
int main(void) {
    printf("%d\n", SQUARE(2 + 3));
    return 0;
}

What is the purpose of an include guard (#ifndef X / #define X / #endif)?

Which of these is NOT a preprocessor directive?