If We Found a Star Cluster at the Edge of the Disk

Multiple Choice

If we found a star cluster at the edge of the disk of the Milky Way at a distance of 10 kpc from the center, which orbited with a velocity of 300 km/s, what would be the total mass enclosed within this radius?

A) 3 * 10¹¹ M _$\odot$ 0
B) 7 * 10¹¹ M _$\odot$
C) 2 *10¹¹ M _$\odot$
D) 4 * 10¹² M _$\odot$
E) 8 * 10¹² M _$\odot$