18. The forest fire caused by the volcano is difficult to be_____. 
A. put off                   B. put away 
C. put out                   D. put up
答案：C 短语辨析。根据题干大火应该是被扑灭的，应选C，put out 扑灭，出版。

19. It was not_____ midnight that they discovered the children were not in their beds. 
A. before             B. at            C. after           D. until
答案：D 固定搭配。It is…that 是一个强调句，可以看出本句强调的是时间。A、B、C、D项都可以和midnight 进行搭配，但能和not 连用表示强调的只有D项，只有….才….。

20. The girl is not happy at the new school. She has _____ friends there. 
A. few               B. a few         C. little           D. quite a few
答案：A 词义辨析。A项是几乎没有的意思，而且few还是修饰可数名词的，其它项都不符合题意，所以应选A。 

21. Robots have already_____ human tasks in the industrial field. 
A. taken on           B. taken out      C. taken up        D. taken over
答案：A 词义辨析。take on 承担，从事。take out 拿出，带出。take up 开始从事，占据。take over接管，接收。根据句意应选A项。 

22. I can’t find the recorder in the room. It ____   by somebody. 
A. must have taken            B. may have taken 
C. may have been taken        D. should have been taken
答案：C 情态动词的推测。首先根据by可以看出应该选的是被动式，首先可排除A和B项，而C项的may 是对现在或过去不太肯定的推测，D项should表示据事实而论，所以根据句意应选C项。

23. He took the medicine, but it didn’t have any_____. 
A. answer            B. cause         C. effect          D. effect
答案：C 词义辨析。Answer 答案。Cause 原因。Effect 作用，影响。Effect 工作。据句意应选C。 

24. It is_____ that over one million Americans now live below the poverty line. 
A. judged            B. estimated      C. calculated      D. considered
答案：B 词义辨析。Judged 审理，断定。Estimated 粗略的计算或由不完全资料计算的；估计，预算，常用It is estimated…这样的句型。Calculated 计算。Considered 考虑。根据句意应选B。