c语言问题

请问这段代码中case 10又什么作用？

#include<stdio.h>

int main(){

int score,grade;

printf("Input a score(0~100):");

scanf("%d",&score);

if(score>100||score<0)  grade=-1;

else                      grade=score/10;

switch(grade){

case 10:

case 9:printf("grade=A");break;

case 8:printf("grade=B");break;

case 7:printf("grade=C");break;

case 6:printf("grade=D");break;

case 5:

case 4:

case 3:

case 2:

case 1:

case 0:printf("grade=E");break;

default:printf("The score is out of range!");

}

}