题目

题型：不详难度：来源：

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。

解析

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.