四则运算的问题

任务，出30道100以内的四则运算，不能有重复
代码
#include <stdlib.h>
#include <stdio.h>
#include <time.h> 
main( )
{
int a=0;
int b=0;
int i,c;
srand((unsigned)time(NULL));
for (i=0;i<30;i++)
{
    a=rand()%100;
    b=rand()%100;
    c=rand()%4;
    printf("%d",a);
    switch(c)
    {
    case 0:
        printf("+%d=\n",b);
        break;
    case 1:
        printf("-%d=\n",b);
        break;
    case 2:
        printf("*%d=\n",b);
        break;
    case 3:
        printf("/%d=\n",b);
        break;
    default:
            printf("错误\n");
            break;
    }
}
return 0;
}
运行结果