Imagine Walking Through a Tropical Rainforest

Imagine walking through a tropical rainforest. You notice that there are different types of trees, birds, insects, and a plethora of other living things. A few weeks later you are taking a walk through the desert and notice that the trees, birds, insects, and many other living things are different than those you saw in the rainforest. Which of the following statements BEST explains the differences between each of these ecological systems?

A) The manner in which organisms interact with each other and their physical environment shapes the diversity found in an ecological system.
B) Organisms that evolved in the rainforest found it easier to live in that ecological system, so they have not spread out to evolve adaptations necessary to live in the desert.
C) Organisms in each ecological system haven't had enough time to evolve the adaptations for the other ecosystem; with enough time they will evolve into the other ecological system.
D) Organisms in each ecological system are there by chance and their presence in different ecological systems does not have a biological explanation.