The rivers will become dirtier and dirtier _____

试题详情及答案解析

The rivers will become dirtier and dirtier ______ we take action to protect them.

A．since

B．if

C．until

D．unless

答案：D

试题分析：句意：如果我们不采取措施保护他们，这条河只会越来越脏。since 自从……以来；until直到；unless除非，如果不。根据句意，故选D
考点：考查连词的用法。