Olivia
Friendships get "destroyed" easily... if you're young and in school, don't expect them to last... they won't. Unless you like try super hard, then they might. And unless they like you a lot and wanna keep the hope that you'll marry them one day, then they might.
Anyways, on the ...
Olivia The thing that worried me the most, has always been destroying friend A & Bs friendship because I choose one.
What will destroy the relationships within your friends is you not deciding... You should be open and clear about your feelings... (realizing my sister will probably be placed in this same situation when she's older so I'll have to copy and paste my argument in like 4-5 years) But let me be clear what I mean about deciding..
I do not mean picking between your friends on who you will date based on their feelings. Which by the way is very selfish of them to put you through that!
I mean deciding whether you even want to be part of the situation in general. Deciding between telling them both to leave it, and telling them that you aren't ready for any of that. Or deciding that you can do it.
By the way, you should talk to them both and ask them how they would feel if they weren't chosen, to understand better. And you should talk to them and explain why you might not pick either, whether you decide to pick one or not because you don't want to leave them asking themselves questions like why they're not good enough.
But I agree with @Flaffy with letting them down with a flat NO if you will tell them no. If you don't care for either or, you should make sure they know that. Sit down and talk to them, 1 on 1, and tell them that you don't have feelings for them like that, and that nothing (possibly) will change that.
If you don't see yourself saying that nothing will change it, perhaps cause you enjoy their friendship and the ability of dating either or as you get older could come up within time if you feel it so, then you should tell them that specifically with the absolute determination that at this time it will not happen.
Most of all, make sure the outcome is something that doesn't leave you asking yourself "was this okay?" "will they be fine?" Make sure you talk to them through it, so that they don't leave the situation feeling like they've lost.
Even though they probably have.