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

问题补充：

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

答案：

解析：句意为：如果你正在一个和你自己的风俗习惯不同的地方旅行，那么你就要入乡随俗。在一个和自己风俗习惯不同的地方旅行，是地点状语从句，故应用where引导，in which=where是在定语从句中的用法, in which是定语从句中的用法。故答案应为D。