The Amount of Visible Matter in a Typical Cluster of Galaxies

Multiple Choice

The amount of visible matter in a typical cluster of galaxies is

A) just sufficient to hold the inner regions of the cluster together, but the fringe galaxies are drifting away from this center.
B) not sufficient to hold the cluster together by a factor of 2, such that the cluster is slowly spreading out.
C) insufficient to hold the cluster together, by about a factor of 10.
D) far more than enough to hold the cluster together gravitationally and is slowly concentrating the cluster into a very tight supercluster.