This is an important problem _______ too few social scientists have researched.
D
intowhich引导的是problem的定语从句。which指代problem，into与research搭配。
The old couple have been married for 40 years and never once _______ with each other.
The conclusion from the study is not definite; it is just ________.
_______ is a fact that English is being accepted as an international language.