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参考答案：错
You should understand the math problem now, you have had it ______ three times. [ ]
A. explaining
B. to explain
C. explained
D. explain
若安全边际率为20%，正常销售量为1000件，则盈亏临界点销售量为（）件。
A.200
B.800
C.600
D.400
