After they had walked a day in the desert,they ___ all tired out.

问题描述:

After they had walked a day in the desert,they ___ all tired out.
A.had been
B.were
C.have been
D.are
选B,请帮我分析一下,谢谢!

这句话是说他们在沙漠里走了一天之后,他们都累垮了.前半句中had walked是过去完成时,后面要接一般过去时的