大神求救

#include <stdio.h>
int main()
{
float score;
char grade;
printf("请输入学生成绩:");
scanf("%f",&score);
while(score>100||score<0)
{
printf("\n输入有误，请重输");
scanf("%f",&score);
}
switch((int)(score/10))
{case10:
case9:grade='A';break;
case8:grade='B';break;
case7:grade='C';break;
case6:grade='D';break;
case5:
case4:
case3:
case2:
case1:
case0:grade='E';
}
printf("成绩是%5.1f,相应等级是%c\n",score,grade);
return 0;
}

为什么等级是输出问号啊？