题目

For most city people, the elevator is an unremarkable machine that inspires none of the enthusiasm or interest that Americans afford trains, jets，and even bicycles. Dr. Christopher Wilk is a member of a small group of elevator experts who consider this a misunderstanding. Without the elevator, they point out, there could be no downtown skyscrapers or tall buildings, and city life as we know it would be impossible. In that sense, they argue，the elevator’s role in American history has been no less significant than that of cars. In fact, according to Wilk? the car and the elevator h如图，AD是⊙O的弦，AB经过圆心O，交⊙O于点C，∠DAB=∠B=30°.小题1:直线BD是否与⊙O相切？为什么？小题2:连接CD，若CD=5，求AB的长.