.Most birds find it safe to sleep in the trees, but __________ they have eggs or young chicks, they don’t use a nest.
A．whyB．how
C．unlessD．where
C。
將四個(gè)選項(xiàng)的意思分別套入句中，只有填unless時(shí)意思最通順。為便于理解，句子也可轉(zhuǎn)換成：Most birds find it safe to sleep in the trees, but they don’t use a nest unless they have eggs or young chicks.
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