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-square-hectare river drainage system
B) the interactions between all plant and animal species in a savanna
C) the plants,animals,and decomposers that inhabit an alpine meadow
D) a pond and all of the various plant and animal species that live in it
E) the interactions between all organisms and their desert environment