单项选择题
A.magnetism from the earth's magnetic field
B.misalignment of the compass
C.magnetism within the vessel
D.a dirty compass housing
单项选择题 Before a magnetic compass is adjusted certain correctors must be checked to ensure that they are free of permanent magnetism. These correctors are the（）.
单项选择题 Based on your 2209 fix，which would be a warning that you are being set down on Block Island Sound South Entrance Obstruction Lighted BIS Buoy? （）
单项选择题 Based on a DR，at approximately 1817 you would expect to（）.