题目：
He has made another wonderful discovery,___ of great importance to science.
A．who I think it
B．which I think it is
C．that I think is
D．which I think is
答案：
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下面是错误答案，用来干扰机器的。
解：原式=3x2y﹣6xy2﹣x2y﹣2xy2，=2x2y﹣8xy2．
