改错They have worked out a method which the agricultural production could be raised on a large scal

题目

改错They have worked out a method which the agricultural production could be raised on a large scal
为什么在which 前加by

答案

（他们想出了一种能让农业产量大幅提高的方法）
这是一句定语从句,先行词是method,关系代词which代替method,但是你不能说（方法农业产量能提高）而应该说（通过这种方法农业产量能提高）by which就表示通过这种方法
简单点：介词+关系代词