Thisisanimportantproblem_______toofewsocialscientistshaveresearched.
D
into which 引导的是 problem 的定语从句。 which 指代 problem ，into 与 research 搭配。
Youngerchildrenarecurious________howthingswork，andmanyofthemwanttotakeaparteverythingwithintheirreach.
_______forthefreeticket,Iwouldnothavegonetoseefilmsooften.
______movesonascaleofthenationallanguage,dialects,andindividualwaysofcommunication.
We________goouttorestaurants,butmostlyweeatathome.
Childrenhaveanatural________abouttheworldaroundthem.