If You Measure the Velocity of a Cloud of Gas

Multiple Choice

If you measure the velocity of a cloud of gas that is rotating around the center of a galaxy in a circular orbit with radius R, you can determine the

A) total mass of the galaxy.
B) mass of the stars and gas in the galaxy.
C) mass of the galaxy enclosed within the radius R.
D) mass of the galaxy located outside the radius R.
E) mass of luminous matter within the radius R.