编辑：
2014-03-26
B.One's mind works fast in time of danger.
C.Sometimes enemies can become good friends.
D.Having too many friends means no friends sometimes.
B
Junior school students in Harbin，Heilongjiang，had a "long weekend".They had four days off from Oct.19-22.It was not for any festival，but was caused by serious smog.
The smog was caused by high levels of PM 2.5 in the air.
PM 2.5 is small particles(微粒) of pollution in the air with a diameter (直径) of 2.5 micrometers or less.Waste gas from cars and factories is the biggest cause of PM 2.5 pollution. On Oct.17，the World Health Organization named air pollution a cause of cancer.
To better protect children，cities are taking action.
On Oct.21，Harbin Education Bureau decided to close primary and junior high schools for two days.
Beijing started a program on Oct.22 which has a warning system with four levels.The red warning is the highest warning level.It means that the next three days will see serious pollution.
When a red warning is issued(发布)，schools will close.
If there's pollution that's not as serious，the Beijing Commission of Education will ask schools to stop PE classes and outdoor activities.
How will students be informed?
标签：预测题
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