Most birds find it safe to sleep in the trees butthey have eggs or young chicks they don’t

问题补充：

Most birds find it safe to sleep in the trees,butthey have eggs or young chicks,they don’t use a nest.

A. why

B. how

C. unless

D. where

答案：

A. why

B. how

(答案→)C. unless

D. where

解析：unless引导让步状语从句,意为“如果不”。根据they have eggs or young chicks和they don’t use a nest可判断出前句是后句的条件,表示“如果它们不下蛋或没有小鸟,它们就不用窝”。why用于引导名词性从句或定语从句,意为“为什么”。how引导名词性从句,意为“如何”,表示动作的方式。where引导名词性从句或定语从句,意为“在……地方”。