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 do biologists recognize communities?

A) Communities have no fixed boundaries; their sizes are simply defined according to what is convenient at a given point in time.
B) Communities can be nested within a larger one; for example, the deer is part of the woodland, which is part of the forest.
C) Communities are characterized by their species composition.
D) All of the above are used to recognize communities.