问题补充：
.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
答案：
A. why
B. how
(答案→)C. unless
D. 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.
