When Springfield High School in Palo Alto，California invited Khan to speak in person－he immediately connected to the students there.

　　The idea of short lessons that can be played over and over again attracted high school senior Bridget Meaney.She said she had trouble with math in the seventh grade.“I think the teachers are good，but they can't teach at a speed that's perfect for everyone，”she said.“I like the idea of learning something in class but then going back and pressing pause or rewind and actually getting a deeper understanding of it.”

　　Originally，Khan kept his lessons short because of YouTube restrictions.Now，he thinks short_is_better.“Education researchers now tell me that 10 minutes is how long someone can have a high level of concentration.And anything beyond that and your brain switches off，”he said.

　　For Khan，teaching math，science，and finance is just the beginning.He says he's ready to expand his YouTube site to include other subjects as well.

　　【语篇解读】 加利福尼亚一个财务分析师放弃自己的工作，而从事一个没有报酬的工作——在网络上教授数学，结果很受欢迎……

　　43．What gave Khan the idea of teaching math online?

　　A．His success in helping his cousin learn math.

　　B．His discovery that many students found learning math difficult.

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