of which,先行词是事、物;你说的题的先行词是one,即the travel agent(旅行社),是物.
of whom,先行词是人
whose先行词和定语从句中的名词有所属关系,
��һֱ��Ϊthe travel agent������~Ҳ�����д���ˣ����д����˵���˼��~~��������ӣ�There are 34 students,7 of ___are boys,�Dz���Ҫдwhom��������ʦ�Ͽξ����ӵ�ʱ��˵7 of which are boys��������