C.
Biotic mean alive, so we can cut off the first 2 easily. Aquatic birds eat frogs, so that one is safe for now. A decrease in something parasitic usually helps the population, so D is out too. The only one left is C.
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!
Explanation:
All organic compounds contain <u>carbon</u>
All organic compounds contain carbon, as well as hydrogen unless it has been replaced by another element
