What Observed Property of the Milky Way Suggests That It

Multiple Choice

What observed property of the Milky Way suggests that it must contain a large amount of matter not in the form of stars, gas, or dust?

A) the rotation curve
B) the velocities of the open star clusters
C) the number and shape of the spiral arms
D) the thickness of the disk
E) the presence of a supermassive black hole at the galactic center