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答案：D
解析：
The students who receive A's, B's, and C's account for (1/5) + (1/4) + (1/2) of the students, that is, 19/20. This leaves 1/20 of the students receiving D's. You know that 10 students receive D's, so 10 is 1/20 of the total. This means that the total number of students is 10 times 20, or 200.
