Most birds find it safe to sleep in the trees, but ________ they have eggs or young chicks, they don

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

答案及解析： C

本题考查连词的用法。unless表“除非”，除非它们下蛋或抚育雏鸟，它们不去使用巢穴。