If you are travelingthe customs are really foreign to your own please do as the Romans do

问题补充：

If you are travelingthe customs are really foreign to your own, please do as the Romans do.

A. in which

B. what

C. when

D. where

答案：

A. in which

B. what

C. when

(答案→)D. where

解析：这里没有地点名词作先行词，可见是由where引导的地点状语从句而不是定语从句，因此不能换为in which。