说明：  利用switch完成程序，并给出一个百分制成绩，要求输出等级A、B、C、D、E。90分以上为A，80—90为B，70—79为C，60—69为D，60分以下为E,负数成绩输出”输入成绩有误”
(Use switch to complete the program, and give a hundred mark system score, require output level a, B, C, D, e. The scores above 90 are a, 80-90 are B, 70-79 are C, 60-69 are D, and below 60 are e. the output of negative scores is "input grade error")