单项选择题

Priming of the tides occurs（）.

A.at times of new and full Moon
B.when the Earth，Moon，and Sun are lying approximately on the same line
C.when the Moon is between first quarter and full and between third quarter and new
D.when the Moon is between new and first quarter and between full and third quarter

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单项选择题

One of the causes of ocean currents is density differences in the water. This is true because （）.

A.in an area of high density the water's surface is lower than in an area of low density
B.surface water flows from an area of high density to one of low density
C.the lesser the density gradient the freer the water is to move
D.it is the density differences that cause the currents to stay in the troughs

单项选择题

Neap tides occur when the（）.

A.Moon is in its first quarter and third quarter phases
B.Sun and Moon are on opposite sides of the Earth
C.Moon's declination is maximum and opposite to that of the Sun
D.Sun and Moon are in conjunction