—What are on show in the museum?
—Some photos _______ by the children of Yushu,Qinghai.
A.have been taken B.were taken C.are taken D.taken
这是一个问与答的.第二句其实是个省略句
相当于
Some photos （which were) taken by the children of Yushu,Qinghai are on show in the museum.
过去分词短语总是作后置定语的.
因为问的是“什么”在展出,所以回答的是“一些照片”什么照片呢?就是“青海玉树孩子们拍的”照片在展出.
过去分词短语充当定语,一般可以换成定语从句.