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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

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出自：海船船员证

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