In ecosystems, why is the term cycling used to describe material transfer, whereas the term flow is used for energy exchange? A) Materials are repeatedly used, but energy flows through and out of ecosystems. B) Both material and energy are recycled and are then transferred to other ecosystems as in a flow. C) Materials are cycled into ecosystems from other ecosystems, but energy constantly flows within the ecosystem. D) Both material and energy flow in a never-ending stream within an ecosystem. E) None of the choices is correct.