Two Planets and a Comet

Think about the question below. Only one of the answers listed is correct. We'll do some (but probably not all) of these questions in the lectures when you will all be asked to vote on which answer you think is correct. (Soon afterwards, you'll be able to see your responses, and those of the whole class at EVAF.)

Two planets of equal mass are orbiting each other. A (very) large comet of identical mass collides with one planet and coalesces. The separation of the two planets increases by a factor two. The ratio of initial to final gravitational potential energy is:
1
4:1
2
2:1
3
1:1