This is really hard and i don't get it you all

The best explanation is B, there used to be salt water. A is unreasonable and unrealistic as sharks and all fishes, excluding dolphins and sometimes whales (if they count as air breathing mammals) can not survive in air for a very long time. C, I think sharks can't survive in fresh water as they can in salt water. D is not very accurate as some people have died from getting a shark attack and getting a shark's tooth is probably a death wish. Hence, B is very accurate as even where I lived, there used to be an ocean.