The subjects don't matter, but give some thought about what your plan b would be. They will ask you about it, should you get to the interview stage.
There is a reason why they tend not to get many 18yr olds in as officers.
Remember, you're applying to be a manager. A leader. You need to show that that's something you can do, that you have the potential of doing that in environments more challenging than anything you've faced. If they think you're not ready for that yet, what are you going to do in the meantime?