Lets say you have to pick between the two. You [i]will[/i] get attacked one way or the other but which would you prefer? You can't just be like "Nuh im 2 cool 4 sherks or allgatres"
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Alligator, it can be on the ground and you might have a chance to runaway whereas sharks are mostly deep in the sea so you'd have to swim really fast to get away.