Question

.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

Answer 1

C。

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.