Tides are superimposed on nontidal rising and falling water levels，caused by（）．

A.speed of the current at a particular time
B.maximum speed of the current
C.direction from which the current flows
D.direction in which the current flows

A.SW'ly direction along the NW coast of Africa
B.S'ly direction off the East Coast of Australia
C.NW'ly direction along the SW coast of Africa
D.SW'ly direction along the SE coast of Greenland

A.the vertical rise or fall of the tide has stopped
B.slack water occurs
C.tidal current is at a maximum
D.the actual depth of the water equals the charted depth