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

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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

单项选择题

Neap tides occur only（）.

A.at a new or full Moon
B.when the Sun，Moon，and Earth are in line
C.at approximately 28-day intervals
D.when the Moon is at quadrature

单项选择题

Neap tides occur（）.

A.at the start of spring，when the Sun is nearly over the equator
B.only when the Sun and Moon are on the same sides of the Earth and are nearly in line
C.when the Sun and Moon are at approximately 90°to each other，as seen from the Earth
D.when the Sun，Moon，and Earth are nearly in line，regardless of alignment order