Question

@Ali2x2 @paki @onePieceFTW

Answer 1

A - The seasons already do this depending on where you are. :P
B - Unless we have regular trade with squirrels and birds I don't think this is the answer.
C- We have plantations for lumber and most of out oxygen is provided by plankton than trees.
D - Looking at how densely populated areas are flooded with rodents and birds I think this is the answer.

Answer 2

I agree with D :)