One of the Big Puzzles About the Properties and Behavior

Multiple Choice

One of the big puzzles about the properties and behavior of large clusters of galaxies is that

A) each cluster appears to consist of the same type of galaxy, some made up totally of spiral galaxies while others contain only ellipticals.
B) they appear to be spread uniformly throughout space in all directions, which is difficult to explain with the Big Bang theory.
C) there appears to be insufficient mass in the luminous matter (stars, and so on) to hold the cluster together gravitationally.
D) they appear not to take part in the general expansion of the universe, in contrast to single separate galaxies, probably because they are gravitationally bound to one another.