程序代码：

#include <stdio.h>
void main()
{
    int score;
    char level = 'D';

    printf("请输入成绩(0~100)：");
    scanf("%d", &score);
    if (score < 0 || score > 100)
    {
        printf("成绩输入错误\n");
        return;
    }
    score = score/5;
    switch (score)
    {
    case 20:
    case 19:
    case 18:
    case 17:
        level--;
    case 16:
    case 15:
        level--;
    case 14:
    case 13:
    case 12:
        level--;
    }
    printf("等级：%c\n", level);
}