Which of the Following Is an Example of an Ecosystem

Which of the following is an example of an ecosystem?

A) all of the brook trout in a 500 hectare² river drainage system
B) the plants, animals, and decomposers that inhabit an alpine meadow
C) a pond and all of the plant and animal species that live in it
D) the intricate interactions of the various plant and animal species on a savanna during a drought
E) interactions between all of the organisms and their physical environment in a tropical rain forest