Any two points A and B on the circle are always at a fixed distance from a fixed point O. The point O is ___.
radius
center
circumference
The fixed point in a circle which is at a fixed distance from any point on the circumference is the center of the circle.
The fixed point in a circle which is at a fixed distance from any point on the circumference is known as the _______ of the circle.
The fixed distance from the center of the circle to any point on the circle is___ .