Hydraulically, servo-operated, automatic, change over valves, utilized in a two ram hydraulic steering gear, serve to（）

A. allow an alternate main pump to start in the fully loaded condition thus developing immediate full torque
B. prevent either main pump from being hydraulically motored when idle by cross pressure flow
C. prevent both units from operating simultaneously which could result in doubling the flow of oil and pressure leading to over pressurization of the system
D. all of the above