As the Figure Below Illustrates, Communities Vary Tremendously in Size

As the figure below illustrates, communities vary tremendously in size from as large as an entire forest to the digestive system of a single deer.
How can biologists recognize where one community ends and a second begins?

A) Communities have no fixed boundaries; their sizes are simply defined according to what is convenient at a given point in time.
B) Each community is nested within a larger one, for example, the deer is part of the woodland, which is part of the forest.
C) Each community can be defined by the movement of food; all the organisms that get their energy from the same group of producers form the community.
D) Communities are human constructs rather than natural entities; as such their boundaries are arbitrarily established.