—Have your read the article A Dirty Beijing, which is about air pollution?
—Yes. It’s highly informative. I no idea that the PM2.5 there sometimes reached 300.
A. have
B. had
C. will have
D. would have
网友回答
A. have
(答案→)B. had
C. will have
D. would have
解析：考查情景交际和时态。句意：--你看过A Dirty Beijing那篇文章了吗？它是关于空气污染的。--是的。真是增长见识啊。我（以前竟然）不知道北京的PM2.5有时能达到300。根据对话情景可知，在看A Dirty Beijing之前，“我”不知道PM2.5达到300这件事，看A Dirty Beijing是过去的动作，故用一般过去时。故选B。