The force exerted by a propeller which tends to throw the stern right or left is called（）.

A.close
B.open
C.switch off
D.switch on

A.only after two hours in the water
B.only after four hours in the water
C.before entry into the water
D.upon entry into the water

A.when approved by the lockmaster
B.when locking downbound
C.at no time
D.during high water only