How gets an #if Preprocessor conditional which contains a variable resolved without producing an error?

Question

How does an #if Preprocessor stage statement which contain variable of compile time get resolved at preprocessor stage itself?

Below is the code which runs without any error:

#include<stdio.h> 

void main()
{
   int num=10; /* compile time */

#if((num%2)==0)  /* #if is preprocessor stage but has num of compile time why not error here? */
   printf("\nNumber is Even");
#else
   printf("\nNumber is Odd");
#endif 
}

Answer 1

For the evaluation in #if, the preprocessor replaces all identifiers that are already defined macros by their appropriate expansion. All identifiers that remain after that have the value 0.