When you read the book, you'd better make a mark______you have some questions.
【正确答案】 D
【答案解析】解析:本题考查状语从句。空格后的句子作为地点状语修饰谓语make,因此填入where。该状语从句易与定语从句混淆,若make a mark后面有成分at the place,则此处可填where或in which,来引导定语从句。本题无此成分,故只能选where来引导地点状语。