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

.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

A. whyB. how(答案→)C. unlessD. where解析：将四个选项的意思分别套入句中，只有填unless时意思最通顺。为便于理解，句子也可转换成：Most birds find it safe to sleep in the trees, but they don’t use a nest unless they have eggs or young chicks.

这个答案应该是对的

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