To use a foreign language effectively you need to understand the culture it is spoken.A.

问题补充：

To use a foreign language effectively, you need to understand the culture it is spoken.

A. what

B. where

C. why

D. which

答案：

A. what

(答案→)B. where

C. why

D. which

解析：句意：为了有效的使用外语，你需要理解语言所说的地方的文化。这句话使用了定语从句，先行词是culture，定语从句中不缺主宾表，缺少的是地点状语，用where引导定语从句，所以选B。