Thisisanimportantproblem_______toofewsocialscientistshaveresearched.
D
into which 引导的是 problem 的定语从句。 which 指代 problem ，into 与 research 搭配。
The engineer ______ my father works is about 50 years old.
I was writing a report last night; ________ I would not have stayed up late.