What Is the Primary Way in Which We Determine the Mass

What is the primary way in which we determine the mass distribution of a spiral galaxy?

A) We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.
B) We calculate its mass-to-light ratio.
C) We apply Newton's version of Kepler's third law to the orbits of globular clusters in the galaxy's halo.
D) We count the number of stars we can see at different distances from the galaxy's center.