The days are gone you can breathe fresh，unpolluted air or drink clean, unpolluted water in the countryside.
A. when
B. that
C. where
D. which
网友回答
(答案→)A. when
B. that
C. where
D. which
解析：考查定语从句。句意：你在农村可以呼吸新鲜无污染的空气、喝到清洁无污染的水的日子已一去不复返了。此处是when引导的定语从句，修饰先行词The days，when在从句中作时间状语，故选A项。