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。

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。