A had leaked B has been leaking C leaked D is leaking

　　for + 一段时间，做时间状语，谓语动词用完成时态。过去完成时要与一般过去时搭配。

　　46. When he arrived, he found _C_ the aged and the sick at home.

　　A nothing but B none other C none but D no other than

　　形容词前加the表示一类人。 none but 只有，仅有。

　　47. The pressure _B_ causes Americans to be energetic, but it also puts them under a constant emotional strain.

　　A competing B to compete C to be competed D having competed

　　the right to vote 选举权。 动词不定式作后置定语要用主动形式。

　　the pressure to compete 竞争的压力。

　　49. As teachers we should concern ourselves with what is said, not what we think _C_.

　　A have to be said B must say C ought to be said D need to say

　　50. Once environmental damage _A_, it takes many years for the system to recover.

　　A is done B is to do C does D has done

　　51. Studies show that the things that contribute most to a sense of happiness cannot be bought, _B_ a good family life, friendship and work satisfaction.

　　A as for B such as C in case of D in view of

　　contribute to 对做出贡献。

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