题目内容：

As we approached the work site,the workers were seen ____the new house.
A) building
B) build
C) built
D) to build
seen是看的过去式吧,那就是工人被.

优质解答

对啊,就是工人被看见正在建房子.
一般这种被动句你如果觉得不太顺,可以把句子还原成主动句,就很容易发现选项.比如这里,你可以还原成： we saw the workers___the new house. 很容易就可以看出来应该选正在进行时吧.