One Important Way Astronomers Can Learn in Some Detail About

One important way astronomers can learn in some detail about what happens when galaxies collide is

A) to watch the Andromeda Galaxy (M31,) which is on a collision course with us, for a decade
B) to look at the supermassive black hole at the center of the Milky Way
C) to examine the satellite galaxies presently orbiting the Milky Way
D) to simulate galaxy collisions on a large computer and watch what the simulation predicts
E) to look at videos of car accidents that are particularly violent