When the ship, its main propulsion device equipped with clutch, arrived at the port and finished with the engine, the correct operation should be______.
A.disengaging the clutch firstly, then stopping the main engine
B.stopping the main engine firstly, then disengaging the clutch
C.stopping the cooling water pump firstly, then stopping the main engine
D.stopping the main engine and disengaging the clutch at the same time