In an adult patient the concentration of an inspired anesthetic can be changed most rapidly when which of the following is used?

(A) A closed system
(B) A circle system with a flow of 2 liters per minute
(C) A circle system with a flow of 6 liters per minute
(D) A to-and-fro system, regardless of the rate of flow
(E) A non-rebreathing system

Please select the best answer.