How Do Astronomers Obtain an Estimate of the Amount of Mass

Multiple Choice

How do astronomers obtain an estimate of the amount of mass that is inside the Sun's orbital path in the Milky Way Galaxy?

A) by counting stars, assuming an average stellar mass, and calculating the total mass
B) by observing the bending of light from distant galaxies as the light passes near the center of the Milky Way
C) by observing the movement of the Galaxy toward neighboring galaxies because of mutual gravitational attraction
D) by applying Kepler's law to the motion of the Sun and other nearby stars