—________ did you stay there?
—I stayed there for two hours.
A:How soon
B:How far
C:How long
D:How often
题目《静夜思》的“思”字的意思是:( )。
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( )