Sharks have highly developed olfactory receptors. The olfactory receptors in animals allows them to sense the odor. Sharks have olfactory receptors more sensitive than humans. They have nostrils located on the underside of their snout, which they use for smelling not for breathing. The smell of the prey animal or the blood of any animal is detected by the shark with the help of these receptors.
Among the options the best answer is "olfactory receptors" - as the sharks have a very good sense of smell. (they have the other receptors, but not as developed) however, they are more famous for their electroreceptrors- they can sense the electric current around them!
Im not really sure about the water, other than the trees wouldn't be absorbing it so there would be extra? But a huge amount of trees like that produce a lot of oxygen, so there would not be less oxygen in the environment. Hope this helps!
