The disinfection of various compartments in a vessel by filling with a gaseous agent to destroy rats and all insects pests which act as germ carriers means（）.

Backing means the wind（）.

A.is changing clockwise in direction
B.is changing anticlockwise in direction
C.is changing cyclonically or variably in direction
D.remains unchanged in direction at the time

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When a feature remains in its existing charted position but has a change of characteristic,（）command is used on a chart.

A.MOVE
B.DELETE
C.AMEND
D.SUBSTITUTE

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