Question

Perhaps the day will come ____ people will be able to breathe clean air in cities．

A．when

B．while

C．as

D．since

Answer 1

A

这道题考查定语从句的用法：when引导定语从句修饰the day，因为在定语从句中缺少时间状语，所以用when。句意是：可能人们在城市里能呼吸新鲜空气的日子就到了。