A:①②③
B:①②④
C:①③④
D:②③④
(几何证明选讲选做题)如图,PT是圆O的切线,PAB是圆O的割线,若PT=2,PA=1,∠P=60o,则圆O的半径r=____
Mr.Li often drives his car to work,but he rides a bike there.