如图PA是圆O的切线，切点为A，PA=2，AC是圆O的直径，PC与圆O交于点B，PB=1，则圆O的半径R=

题干

如图PA是圆O的切线，切点为A，PA=2，AC是圆O的直径，PC与圆O交于点B，PB=1，则圆O的半径R=（　　）

A：2

B：3

C： $\sqrt{2}$

D： $\sqrt{3}$

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D

Accident	A car crashed into a tree. Some people were Trapped(卡住) in the car.
Weather conditions	There was a heavy rain.
Date of call	May 3， 2008
Time of call	9：30 p.m.
Name of caller	Mrs. Green
Place	Zhongshan Road， Nanjing
Conditions of victims	Mr. Green hurt his head. Mrs. Green hurt her left leg. Their daughter felt frightened.
Action	Policemen arrived at 9：40 p.m. and sent the victims to Gulou Hospital at 9：45 p.m.