A
than也可以用作关系代词,引导定语从句【词义相当于that】,但主句中必须有比较级字样和than前后呼应.例如：
He smoked more (fewer) cigarettes than (he bought.) were normally available.他吸的香烟比他买的（弄到的）要多（少）.这里than在语法功能上相当于that/which的功能,但是这里不能改成后者,因为主句中有比较级more.
请再看下列的例句：
We often advise him not to drink more wine than is good for his health.我们经常劝他为了身体健康不要喝过量的酒.
He often sat in a small bar drinking considerably more wine than was good for his health.他经常坐在小酒馆里,喝过量的酒.这不利于他的身体健康.
To understand the situation completely requires more thought than has been given thus far.要彻底理解这种局势需要作比迄今为止更多的思考.