程序代码：

#include <stdio.h>
int main(void)
{
    int cj;
    printf("Please enter the performance(0~5):");
    while(scanf("%d",&cj)==1)
    {
    if(cj>5||cj<0)
    printf("Pleas enter the true performance(0~5):");
    else
    {switch(cj)
    {case 5:printf("The grade level is A.\n");
           break;
    case 4:
    case 3:printf("The grade level is B.\n");
           break;
    default:printf("The grade level is C.\n");}
    printf("Please enter the performance(0~5):");}
                             }
    system("pause");
    return 0;
}