D the peasants were
Among the workers who built the stadium
在建体育场的工人之中,
介词短语,不是句子主要成分,
the peasants 才是主语,
the peasants used to live in the countrside
农民们过去住在乡下
这是原句.
用过去式是因为前面的：
(Among the workers who built the stadium )也用了过去式
在所给句子中,后面用了定语从句.