This is an important problem _______ too few social scientists have researched.
A、whichD
intowhich引导的是problem的定语从句。which指代problem，into与research搭配。
Childrenhaveanatural________abouttheworldaroundthem.
Countrylifeisbetterthancitylife________itoffersfreshairandnoiselessenvironment
Johnsonwas________unknownbeforerunningforthepresidency.
Theoldcouplehavebeenmarriedfor40yearsandneveronce_______witheachother.
Thevolunteerswouldrathergobytrainthan________.
