The number of students is larger than that of those three years ago.
由于number是单数,所以用that代替;后面的students是复数,所以用those代替.如果直接用that来指代,则关系混乱——numer指数目,students指学生,二者没有办法用一个代词同时代替.
其实,这一类的问题都很多.例如,比较级必须是相同的事物、类型等来比较,但学习者在运用时往往会出错.譬如,有学生可能就会说:* You are older than my father's age.
(错句哟!)