要求统计给定整数MM和NN区间内素数的个数并对它们求和

#include<stdio.h>
#include<math.h>
int main()
{
    int m,n,i;
    int cnt=0;
    int sum=0;
    scanf("%d %d",&m,&n);
     if(m == 1)
         m = 2;
     for(i=m;i<=n;i++){
         int c,d;
         d=sqrt(i);
         for(c=2;c<=d;c++){
             if(i%c==0)
             break;
         }if(c>d){                 //为什么这样就能判定是素数？   
             cnt++;
          sum += i;
         }
     }
     printf("%d %d",cnt,sum);
     return 0;
}

c>d说明右边循环已经充分循环,从c到d没有一个数能除断i ,然后i就是素数了   for(c=2;c<=d;c++){
                                                                        if(i%c==0)
                                                                         break;}