有三种情况：介词取决于先行词的名词,从句中的动词,从句中的形容词
给个例句 Gun control is a subject _____ Americans have argued for a long time.(about which--- argue about sth)
There is no one _____ she can turn when in trouble (to whom----turn to sb for help)
The reason _________ he was late was that he got up late(for which----- the reason for)