请帮忙解释一个介词BY的用法

题目

请帮忙解释一个介词BY的用法
An important factor in a market-oriented economy is the mechanism by which consumer demands can be expressed and responded to by producers.
请问为什么需要这句话中的BY,这个BY是与什么对应的,删去这个BY可否.

答案

不能删去,"which"前的BY是指消费者的需求可以通过这种mechanism（结构）表达和由生产者response(反映）出来,表示方式,方法,固不能省去.原定语从句可以还原为consumer demands can be expressed by the mechanism and consumer demands can be responsed to by producer by the mechanism.