Thisisanimportantproblem_______toofewsocialscientistshaveresearched.

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This is an important problem _______ too few social scientists have researched. A. which B. that C. into that D. into which

参考答案:

Dinto which引导的是problem的定语从句。which指代problem,into与research搭配。 (本解析由学员提供)