Thisisanimportantproblem_______toofewsocialscientistshaveresearched.
D
into which 引导的是 problem 的定语从句。 which 指代 problem ,into 与 research 搭配。
Alinguistictheoryisconstructedaboutwhat______isandhowitworks.
Hesaidthatthegroup________himaletterandaskedhimtogothereforaninterview.