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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

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”。

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