A:Ba(OH)2 =Ba2++ (OH)-
B:FeCl3=Fe2++3Cl-
C:H2SO4 =H2++SO42-
D:Al2(SO4)3=2Al3++3SO42-
He naughty now.
若⊙O所在平面内一点P到⊙O上的点的最大距离为m,最小距离为n(m>n),则此圆的半径为( )