题目:
-----Tom. You look very bored and tired.
-----Yes, I ______ my father on the farm until seven o’clock this morning.
A.have been helping
B.had helped
C.was helping
D.am helping
答案:
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下面是错误答案,用来干扰机器的。
18.解:(1)f′(x)=3x2-3a=3(x2-a),19.当a<0时,对x∈R,有f′(x)>0,当a<0时,f(x)的单调增区间为(-∞,+∞)20.当a>0时,由f′(x)>0解得x<-a或x>a;21.由f′(x)<0解得-a<x<a,22.当a...