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