At one time we had about eighty people here who did nothing but ___ into his family history.(D)

题目

At one time we had about eighty people here who did nothing but ___ into his family history.(D)
A.to research B.researched C.researching D.research

答案

do ...but do
前面不是do而是其他动词的话就是 动词..but to 动词
以前我们大约有八十来个人不做其他事,只是调查他的家庭史