A 33-year-old financial analyst in California recently quit his job to devote himself to an unpaid job teaching math on the Internet, and his lessons are reaching almost 100，000 people a month. Sal-man Khan's voice is heard every day on the net—by tens of thousands of students around the world who are hungry for help in learning math. He has posted 1，200 lessons on YouTube—lessons that appear on an electronic blackboard, which range from basic addition to advanced mathematics for science and finance. And they are free.
Khan lives in Silicon Valley, with his wife, a doctor, and their new baby. He got the idea for his “Khan Academy" four years ago, when he taught a young cousin how to convert (转换）kilograms to grams. With Khan's help, the cousin got good at math, and Khan began a new career.
Now, Khan records his lessons himself, but he never goes on camera. “It feels like my voice in their head. You're looking at it and it feels like someone's over your shoulder talking in your ear, as opposed to someone at the blackboard, which is distant from you，” he said.
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 says 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 (倒回）and actually getting a deeper understanding of it. ”
Originally, Khan kept his lessons short because of YouTube restrictions (限制）. 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.
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.

C．A suggestion made to him at a local high school.

D．His interest in Internet teaching.

Why does Khan never go on camera?

A．He's too shy to show his face on camera.

B．It's restricted by YouTube for education videos.

C．He wants to keep distant from the viewers.

D．He wants to create a more relaxed learning atmosphere.

Why does Bridget Meaney like Khan's lessons?

A．Khan teaches seventh grade math better than her teacher.

B．The lessons can be watched repeatedly until fully understood.

C．She can perfectly follow the pace of Khan's teaching.

D．She cannot concentrate when learning in class.

From the passage, we know that __________.

A．Khan travels to many schools to promote his lessons

B．Khan plans to include more subjects in the future

C．Khan gives live math lessons every day for free

D．Khan set up the Khan Academy with his wife