 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? "First，the Commission of Education will inform schools online，" Ye Yan，head of Beijing Shijingshanqu Experimental Primary School，told Beijing Times．"Then teachers will text students’ parents．" Will the days off affect the study plan? "Missing school days will be made up during the summer or winter vacations，" said Wang Hao，a physical education teacher in Beijing Chaoyang district． For students in Harbin who had the long weekend，they would go to school on the next two Saturdays． 根据材料内容选出最佳选项，并在答题卡上将该项涂黑。 小题1:Why did students in Harbin have a long weekend?
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