Perhaps the day will come people will be able to breathe clean air in citiesA. whenB. wh

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(答案→)A. when
B. while
C. as
D. since
解析：
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。