A Graduate Student in Astronomy Needs to Measure the Mass

A graduate student in astronomy needs to measure the mass of a spiral galaxy she is studying for her PhD thesis. Which of the following observations would be important for her to make?

A) determine whether or not there is evidence for a massive black hole at the galaxy's center
B) construct an H-R diagram for a prominent open cluster in the galaxy's disk
C) measure the gamma-ray emission from the galaxy
D) compare the overall color of the galaxy to other galaxies of the same type
E) obtain the speed at which stars or gas near the outer regions of the galaxy are moving around