Most birds find it safe to sleep in the trees, but ____ they have eggs or young

题型：不详难度：来源：

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

A．why

B．how

C．unless

D．where

答案

C

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”。