Why should galaxy collisions have been more common in the past than they are today?

a. galaxies attracted each other more strongly in the past because they were more massive; they had not yet turned most of their mass into stars.

b. galaxies were much bigger in the past since they had not contracted completely.

c. galaxies were closer together in the past because the universe was smaller.

d. galaxies were more active in the past and therefore would have collided with each other more frequently.