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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:

由they have eggs or young chicks与they don’t use a nest之间的逻辑关系可以推出该空表示“除非”,因此应填unless引导条件状语从句。

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