1.— There’s someone knocking at the door.Who could _____ be?

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1.— There’s someone knocking at the door.Who could _____ be?
— I have no idea.
A.he B.it C.she D.that
为什么答案是B,D为什么不对?
2.At one time we had about eighty people here who did nothing but_____into various family history.
A.researched B.research C.researching D.to research
答案为什么是B,而不是C?
3._______ harder,you would have passed this exam.
A.If you have worked B.If you worked C.Were you to work D.Had you worked
答案为什么是D而不是A?
4.Your hair needs _________.You’d better have it done tomorrow.
A.cut B.cutting C.to cut D.being cut
D选项为什么不对?
5.When _______ to dinner at the White Rose Restaurant,he seemed very happy and accepted the invitation.
A.invited B.inviting C.to be invited D.having invited
为什么C不对呢?

1.因为听见敲门声,你并不知道是男是女,在英语习惯中,用it 代替.接电话也是一样的,通常会说:"who is it?"
2.时态一致.but 前用的是did ,所以but 后应该用researched.
3.这是一个省略句.还原后应该是"if you had worked".后面跟的是"would have"前面应该用“过去的过去”时态,所以用"have worked "不对.
4、need后要么接不定时,要么接动词原形,不接v.+ing时态
5、when后面不能接不定式.

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