Which Single Major Problem Perhaps Puzzles Astronomers MOST as They

Multiple Choice

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

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