#include<stdio.h>
void main()
{
int a,b;
char ch;
printf("please scanf a's value:");
scanf("%d",&a);
b=a/10;
switch(b)
  {
     case 5: ch='E';//case和5之间要有空格
            break;
     case 6: ch='D';
            break;
     case 7: ch='C';
            break;
     case 8: ch='B';
            break;
    case 9: ch='A';
           break;
     default:
        break;
  }
printf("This student's grade is:%c\n",ch);
}

case和数字之间要有空格