The center of a sphere is
[1] a line segment from the center point to the surface of the sphere.
[2] a fixed point equidistant from all points on the surface of the sphere.
[3] a three-dimensional circle in which all points are equidistant from a fixed point.
the same as the base of the sphere.

Look at the figure below: triangle abc is a right triangle with angle abc equal to 90 degrees. the length of ac is 5 units and the length of ab is 4 units. d is a point above c. triangle adc is a right triangle with angle dac equal to 90 degrees and dc parallel to ab. what is the length, in units, of segment cd?