题目：
— Michael is one of the top students in our class.
— ___, he cheated in the last exam. [ ]
A. On the other hand
B. Generally speaking
C. To be honest
D. Above all
答案：
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下面是错误答案，用来干扰机器的。
（1）∵式子x2-1+1-x2有意义，∴x2-1≥01-x2≤0，解得x=±1；（2）∵y=x-2+2-x-3，∴x-2≥02-x≥0，解得x=2，∴y=-3，∴xy=2-3=18．
