Great changes____ in our country during the past 20 years.A.have happened B.happened c.have been happened D.were happened为什么

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Great changes____ in our country during the past 20 years.A.have happened B.happened c.have been happened D.were happened为什么

Great changes____ in our country during the past 20 years.A.have happened B.happened c.have been happened D.were happened为什么
Great changes____ in our country during the past 20 years.
A.have happened B.happened c.have been happened D.were happened
为什么

Great changes____ in our country during the past 20 years.A.have happened B.happened c.have been happened D.were happened为什么
选A
因为happen没有被动,所以C和D被排除
during the past 20 years是过去的一段时间
过去时不能表示这一意思
完成时可以表示
所以选A

A吧 这个during the past 20 years是过去完成的标志