Our neighbor offered to look after our baby yesterday when we were busy is often the case

问题补充：

Our neighbor offered to look after our baby yesterday when we were busy, is often the case.

A. it

B. that

C. which

D. as

答案：

A. it

B. that

C. which

(答案→)D. as

解析：考查定语从句。这里考查由as引导的非限定性定语从句，as在句中由“正如”之意。句意：昨天当我们都很忙的时候，我们邻居主动来照顾我们的婴儿，通常就是这样的。故D正确。