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问题 选择题

16. He made another wonderful discovery, __________ of great importance to science.

A．I think which is

B．which I think it is

C．which I think it

D．which I think is

答案

答案：D

解答题

（1）写出命题：“若x2-3x+2=0，则x=1或x=2”的逆命题、否命题和逆否命题，并判断它们的真假；

（2）已知集合P={x|-1＜x＜3}，S={x|x2+（a+1）x+a＜0}，且x∈P的充要条件是x∈S，求实数a的值．
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选择题

There may be a(n)_________for you to visit the Great Wall with Mr. Bush tomorrow. [ ]

A. change        

B. opportunity  

C. idea            

D. time
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