用计算器求一组数据21,22,25,23,27,19,24,20,25,24,18,27的平均数是(保留一位小数)( )
(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=____
A. doesn't B. didn't C. haven't D. hasn't