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代码说明：

说明：  按分段统计学生成绩：要求按 90－100、80－89、70－79、60－69、60以下五档分别统计各分数段人数，输入为负数时结束。（分别用if-else和switch）(According to the partition of student achievement statistics: according to the requirements of 90 to 100, 8089, 70-79, 6069, 60 the following five respectively statistics the number of segments in each fraction, input is negative end. (respectively if-else and switch))