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