21.(C).Finding prime number

 #include<stdio.h>

// finding whether a number is prime or not

/* Prime number are those which are not divisible by any number except

  themeselves*/

int main(){

    int num,i,count=0;

    scanf("%d",&num);

/* for loop will help to produce 2 or bigger natural number than 2

   but not greater than num.Then,if statement will test  if

   the number is divisible by any number .If it's true then count wil increase

   one time and leave the loop*/

   

    for(i=2;i<num;i++)

    {

        if(num%i==0)

        {

            count++;

            break;

        }

    }                //count will be 0 if it is not divisible

    if (count==0){

        printf("prime\n");

    }

    else

    {

        printf("not prime\n");

    }

   

    return 0;

}

/* 2 is a prime number but it is equal to i. So, it not satisfying for the loop

condition thus it will not enter into the loop. As the count is set to

0 the count for 2 is also 0. As a result, it will enter in last if condition

and satisfying the condition it will print 2 as prime*/