（）sends a radio signal from the bottom of the ship to the sea-bed,from which it is reflected.

单项选择题（）is the navigational timekeeper of the vessel.

A.The marine sextant
B.The chronometer
C.The magnetic compass
D.The gyrocompass

单项选择题（）is used for measuring horizontal and vertical angles.

A.The marine sextant
B.Thechronometer
C.The magnetic compass
D.The gyrocompass

单项选择题（）means rudder to be held in the force and aft angles.

A.Steady
B.Meet her
C.Midships
D.Stop engines

单项选择题（）means check the swing of the vessel’s head in a turn.

A.Steady
B.Meet her
C.Midships
D.Stop engines

单项选择题1 nautical mile is the lenth of an arc of a great circle which subtends an angel of（）at the center of the Earth.

A.1degree
B.1 minute
C.1second
D.1.5 degree