[A] An ecosystem is a functional unit where living organisms interact only among themselves without involving the physical environment.

[B] An ecosystem can be of any size, includes specific and limited species, and involves interaction between biotic and abiotic components through nutrient cycles and energy flows.

[C] An ecosystem consists only of living organisms and does not depend on abiotic factors like soil or climate.

[D] An ecosystem is a large geographical area that has no interdependence between species and physical components.