If it were possible to go back in time, then it would be impossible to go back in time with the intention of doing anything. Doing so would create a paradox which, according to some theorys' could, destroy the universe, fade you out of existance, make something really really bad happen, or just not be able to accomplish what you want.
The reason this is so is that at point A, you want to go back in time to get rid of bunny's. So you go back to point B, kill all the bunny's and Boom, there goes the universe. Becuase, when history continues from point B to point A, (now C) you will no longer want to go back and kill bunnys becuase there are no such things as bunnys. Thus creating a Paradox.
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