问题
选择题
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
C。
将四个选项的意思分别套入句中，只有填unlessunless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.