Which Single Major Problem Perhaps Concerns Astronomers the Most as They

Multiple Choice

Which single major problem perhaps concerns astronomers the most as they attempt to interpret the properties and behavior of clusters of galaxies?

A) The presence of star formation in many galaxies,long after it is expected to have died out
B) The problem that at least 10 times more mass is needed for galactic cluster stability than is observed in visible material
C) The structure and motion of spiral arms in galaxies in many clusters
D) The excessive rate of supernova occurrence in galaxies in some clusters