Collisions Between Galaxies

Collisions between galaxies:

A) occur only very occasionally to produce spectacular supergalaxies containing many new stars. Only one or two examples have been detected so far.
B) are occurring all the time, all elliptical galaxies having resulted from such collisions. The collisions have cleared out all the gas and dust from these galaxies.
C) never happen since galaxies are so far apart in our universe.
D) can and do occur relatively frequently over cosmological times.