How does the below program output `C89` when compiled in C89 mode and `C99` when compiled in C99 mode?

C99 allows //-style comments, C89 does not. So, to translate:

C99:

 printf("C%d\n",(int)(90-(-4.5     /*Some  comment stuff*/
                         -4.5)));
// Outputs: 99

C89:

printf("C%d\n",(int)(90-(-4.5/      
                         -4.5)));
/* so  we get 90-1 or 89 */