The smog arrived in the Yangtze River Delta region on Nov．30， the air quality in 1 2 cities out of 25 was rated as severely polluted by Dec．1．
A. where
B. when
C. which
D. what
网友回答
(答案→)A. where
B. when
C. which
D. what
解析：句意：雾霾在11月30号到达长三角区域那里25个城市中有12个被列为严重污染。这里用了定语从句，先行词是the Yangtze River Delta region，定语从句中不缺主宾表，缺少的是地点状语，所以用where引导定语从句，选A。