Rotation Curve for a Spiral Galaxy is "Flat"

Question

What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?

Answer

A) The amount of light emitted by stars at different distances is about the same throughout the galaxy.

 B) Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.

C) The disk of a spiral galaxy is quite flat rather than spherical like the halo.

D) All the galaxy's mass is concentrated in its flat, gaseous disk.