This is because it is believed that the galaxy was once crammed into a tiny spot and the Big Bang explosion sent the universe expanding and galaxies hurtling way into space away from each other. The force from the Big Bang that is believed to cause this expansion is dark energy.
While all the mentioned phenomena do take place in the universe, the only one that provides evidence of the Big Bang is option D, where distant galaxies are moving further away from us as time goes on.
This is believed to be a result from the Big Bang that projected the known universe into all directions, and they are moving away because of momentum from the explosion so many years ago.
In this case, we have a dilution problem. We have to remember that in the dilution procedure we go from a solution with higher concentration to a solution with lesser concentration. Therefore we have to start with the dilution equation:
