求助！c语言问题，

随机出十道100以内计算题，要求加减乘除概率相等
出现的问题：有的时候运行到一半就不行了。
希望大神帮帮忙
#include <stdio.h>
#include <ctype.h>
#include <time.h>
#include <stdlib.h>
#include <windows.h>
#define IDNO_LENGTH 6
#define CALC_KIND 4
#define CALC_NUM 10

bool checkNo(char idNo[]);
char getSignal();
int getResult(int,int,char);
void takeTest();

//main函数
int main()
{
printf("Please input your four digit ID no(begin with two letters):");
char idNo[IDNO_LENGTH];
do
{
scanf("%s",&idNo);
fflush(stdin);
} while(checkNo(idNo)==false);

printf("1.Start a test\t2.Check score\t3.Exitb\n");
int choose;
do
{
printf("选择：");
scanf("%d",&choose);
fflush(stdin);
} while(choose!=1 && choose!=2&& choose!=3);

if(choose==1)
{
takeTest();
}else if(choose==2)
{
FILE *fp;
char ch;
if((fp=fopen("score.txt","w+"))==NULL)
{
printf("\n你还没做过题目测试");
getchar();
exit(1);
}
ch=fgetc(fp);
while(ch!=EOF)
{
putchar(ch);
ch=fgetc(fp);
}
fclose(fp);
}
else
{
exit(0);
}
}

//获取随机运算符
char getSignal()
{
char signal[CALC_KIND]={'+','-','*','/'};
srand((unsigned)time(NULL));
return signal[rand()%4];
}

//获取随机数
int random(void)
{
int i;
i=rand() % 100;
return (i);
}


//运算
int getResult(int num1,int num2,char signal)
{
float res;
switch(signal)
{
case '+':
res=num1+num2;break;
case '-':
res=num1-num2;break;
case '*':
res=num1*num2;break;
case '/':
res=num1/num2;break;
default:
res=0;
printf("运算符不符合规则\n");
}
return res;
}

//int checkresult(int h)
//{
// float res;
// res=getresult(num1,num2,signal);
// if (int(res)!=res|res>=100) h=0;
// else h=1;
// return h;
//}

//列出10道题
void takeTest()
{
printf("开始答题，每道题10分，总共100分\n");
clock_t startTime;
startTime=clock();
int index;
int res;
int giveRes;
int l,k,h;
int num1;
int num2;
int score=0;
char signal;
for(index=0;index<CALC_NUM;index++)
{
signal=getSignal();

srand((unsigned)time(NULL));
if(signal=='+'||signal=='-')
{
num1=random();
num2=rand()%(100-num1);
}
if(signal=='-')
{
num1=random();
num2=rand()%num1;
}
res=getResult(num1,num2,signal);
if(res>100||res<0) k=0;
else k=1;
if(signal=='*')
{
num1=random();
num2=rand()%(100/num1);
}
if(signal=='/')
{
do
{
num1=random();
num2=rand() % num1;
int c;
c=num1/num2;
if(num2*c==num1) h=1;
else h=0;
}while(h==0);
}

printf("%d%c%d=",num1,signal,num2);
scanf("%d",&giveRes);
fflush(stdin);
if(getResult(num1,num2,signal)==giveRes)
{
printf("correct\n");
score+=10;
}
else
{
printf("wrong\n");
}
}
FILE *fp=fopen("score.txt","w");
fprintf(fp,"分数为%d",score);
fclose(fp);
printf("解10道题用了%lf秒\n",(double)(clock()- startTime)/CLOCKS_PER_SEC);
}

//检测idNo是否合规则，合规则返回true
bool checkNo(char idNo[])
{
int index;
bool flag;
flag=true;
for(index=0;index<IDNO_LENGTH;index++)
{
if(index<2)
{
if(!isalpha(idNo[index]))
{
flag=false;
printf("ID no 不符合规则，重新输入：");
break;
}
}
else
{
if(!isdigit(idNo[index]))
{
flag=false;
printf("ID no 不符合规则，重新输入：");
break;
}
}
}
return flag;
}