She had three sons,all of whom became doctors.这里面的连词为什么用whom,从句中缺少主语,不是wo og

She had three sons,all of whom became doctors.这里面的连词为什么用whom,从句中缺少主语,不是wo og
从句中缺少主语,不是应该用who么,如果是whom的话,它在句子中作什么cheng dem