單項(xiàng)選擇題Scouring is the result of（）.

A.interruption of the normal current flow by the footing
B.elevating in mud-slide zones
C.penetrating rapidly through a hard crust into weaker soils
D.extracting legs from soils with high cohesion


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1.單項(xiàng)選擇題River currents tend to（）.

A.pick up speed where the channel widens
B.run slower in the center of the channel
C.hug the inside of a bend
D.cause the greatest depth of water to be along the outside of a bend

2.單項(xiàng)選擇題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

3.單項(xiàng)選擇題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

4.單項(xiàng)選擇題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

5.單項(xiàng)選擇題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