如图，一条公路的转弯处是一段圆弧（图中的弧AB），点O是这段弧的圆心，C是弧AB上一点，OC⊥AB，垂足为D．若这段弯路的半径是100m，CD=20m，则A、B两点的直线距离是（　　）

    Xiao Qing is my friend. He is thirteen years old. He is in a big school. There is a playground in the middle of his school. The red classroom building is on its left and on its right is the library. Behind the playground is the dining hall. Xiao Qing and his classmates have lunch there from Monday to Friday. Every day they can eat meat. Today is Friday. They eat beef .There is soup, on Wednesday. It's Xiao Qing's favourite. Basketball is his favourite sport. He plays it on Tuesday and Thursday afternoon. But his classmates play football and jianzi. Today is Xiao Qing's birthday. He wants a football. So he can play it with his classmates.