---How do you learn English so well ?- chatting with my uncle in the USA.A. for B. By C

问题补充：

---How do you learn English so well ?

--- chatting with my uncle in the USA.

A. for B. By C. In D. With

答案：

解析：句意为：---你怎么英语学得那么好？---（我是）通过和在美国的叔叔聊天学的。how提问一种方式方法，by短语表示通过某种方式、手段。故应选B。