There are many people downstairs. What do you think ?A. to happenB. happeningC. is happene

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A. to happen
B. happening
C. is happened
D. has happened
解析： 注意现在完成时和一般过去时两种时态的区分
(1)一般过去时的谓语动词用过去式，而现在完成时的谓语基本构成是助动词have /has +过去分词。 (2)一般过去时通常与表示过去的时间状语连用。如：yesterday, last week, two years ago, just now, in 2002等； 而现在完成时则常与just, already, ever, never等副词和these days, this week, since..., for...等表示一段时间的状语连用。