He was lucky enough to sell his bicycle for exactly -----为什么

这里无论用“what”还是“which”该从句都只能是介词For的宾语从句,属于名词性从句。因此以下我们要讨论的就是what和which在引导名词性从句是的区别,以判定在这句话中究竟应该用what还是which:what引导的名词性从句有两种用法：1。疑问代词,具有疑问含义,如：I don't know what his name is。我不知道他叫什么名字。（什么）2。关系代词型what,相当于一个名词加一个定语从句如本句中的what he had paid for it= the money that he had paid for itwhat=the money that而which所引导的名词性从句,which具有疑问的含义,“哪一个”,因此在此不可用。which在这里引导的不是定语从句,因此不能作其它的解释。