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气孔是植物叶片新陈代谢过程中气体进出的通路.光照是影响气孔开闭的主要因素,一般情况下,气孔在光照下开放,在黑暗中关闭.适当提高温度也能增加气孔的开放度,但是如果叶片蒸腾作用强烈时,即使在适宜光照和温度条件下气孔也要关闭.某科研小组在夏季晴朗的白天,测定了6:00﹣18:00间黄瓜幼苗叶片的气孔阻力(气孔开度减小,气孔阻力增大),结果如图所示.回答下列问题:

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   About 21,000 young people in 17 American states do not attend classes in school buildings.
   Instead, they receive their elementary and high school education by working at home on computers. The center for Education Reform says the United States has 67 public “cyberschools(网络学校)” and that is about twice as many as two years ago.
   The money for students to go to a cyberschool comes from the governments of the states where they live. Some educators say cyberschools receive money that should support traditional public schools. They also say it is difficult to know if students are learning well.
Other educators praise this new form of education for letting students work at their own speed. These people say cyberschools help students who were unhappy or unsuccessful in traditional schools. They say learning at home by computer ends long bus rides for children who live far from school.
   Whatever the judgment of cyberschools, they are getting more and more popular. For example, a new cyberschools called Commonwealth Connections Academy will take in students this fall. It will serve children in the state of Pennsylvania from ages five through thirteen.
Children get free equipment for their online education. This includes a computer, a printer, books and technical services. Parents and students talk with teachers by telephone or by sending emails through their computers when necessary.
   Students at cyberschools usually do not know one another. But 56 such students finished studies at Western Pennsylvania Cyber Charter School recently met for the first time. They were guests of honor at their graduation.