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
网友回答
A. why
B. how
(答案→)C. unless
D. where
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。