How Do Astronomers Measure the Mass That the Galaxy Contains

Multiple Choice

How do astronomers measure the mass that the Galaxy contains inside the orbit of the Sun?

A) they count the number of stars one by one and multiply by the average mass of a star
B) they add up all the observations at 21-cm wavelengths (because the mass of hydrogen gas is far greater than the mass in all the stars)
C) they measure the distance to the center of the Galaxy and the period of the Sun's orbit and then use Kepler's Third Law
D) they measure the masses of many other spiral galaxies nearby and then take an average
E) you can't fool me; there is no way to get the mass of something as big as our Galaxy