自动生成题的四则运算

#include <stdio.h>
#include <time.h>
int main()
{
    int a,i,n,o,x,y,z;
    printf("请输入题目数：");
    scanf("%d",&n);
    while(n--)
    {
        srand((unsigned)time(NULL));
        x=rand()%10+1;
        y=rand()%10+1;
        o=rand()%4;
        switch(o)
        {
            case 0:
                printf("%d + %d = ",x,y);
                a=x+y;
                break;
            case 1:
                printf("%d - %d = ",x,y);
                a=x-y;
                break;
            case 2:
                printf("%d * %d = ",x,y);
                a=x*y;
                break;
            case 3:
                printf("%d / %d = ",x,y);
                a=x/y;
                break;
        }
        scanf("%d",&z);
        i=2;
        while(z!=a&&i--)
        {
            printf("答案错误，输入正确后才可获取下一题!\n");
            scanf("%d",&z);
        }
        if(z==a)
            printf("答案正确!\n");
    }
    return 0;
}