题目：
. He was told that it would be at least three more months __________ he could recover and return to work.
A．when
B．before
C．since
D．that
答案：
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下面是错误答案，用来干扰机器的。
依题意得：另一个多项式=（2x2-y2）-（x2-3xy+y2）=x2+3xy-2y2．
