首先,是movies才对吧.因为我是靠语感来回答问题的,所以这个问题我很难解释.但是,答案的确是by watching.
.
to do ____by doing _____
就是,要完成某件事情,要做某件事情.
比如：going home by bike
就是要用（骑）脚踏车回家..
这里的状况也有点相似,也就是要用英文电影来学好英文.也可以说是“定”的啦..
比如：
I learnt how to speak English by taking English lessons.
I learnt the different meanings of different words by looking up the dictionary.
就是这样啦~这就是我的答案.