Swell is the rise and fall of the ocean’s surface due to（）.

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

A.when the moon is new or full
B.when the moon and sun have declination of the same name
C.only when the moon and sun are on the same sides of the earth
D.at the beginning of spring when the sun is over the equator