A.must be from dead ahead
B.speed must be zero
C.must be from dead astern
D.must be on the beam
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A.ahead and equal to the ship's speed
B.astern and equal to the ship's speed
C.ahead and equal to twice the ship's speed
D.astern and equal to twice the ship's speed
A.spring，around late evening
B.spring，around early evening
C.fall，around early morning
D.fall，around midday
A.always forms over water
B.is formed by a temperature inversion
C.is thinnest at the surface
D.dissipates during the evening
A.prevailing westerlies
B.geostrophic winds
C.doldrums
D.horse latitudes
A.create large waves in the direction of the current
B.change direction 360°during a 24 hour period
C.remain fairly constant in direction and velocity throughout the year
D.are characterized by a light green color
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In modern fathometers the sonic or ultrasonic sound waves are produced electrically by means of a(n)（）.
The drift and set of tidal，river，and ocean currents refer to the（）.
In some river mouths and estuaries the incoming high-tide wave crest overtakes the preceding low-tide trough. This results in a wall of water proceeding upstream，and is called a（）.
On the pole side of the trade wind belt，there is an area of high pressure with weak pressure gradients and light，variable winds. This area is called the（）.
In some parts of the world there is often a slight fall in tide during the middle of the high water period. The effect is to create a longer period of stand at higher water. This special feature is called a(n)（）.
The apparent wind’s speed can be zero only when two conditions are present. One condition is that the true wind（）.
Steam fog is most likely to occur on the Mississippi and Ohio Rivers in（）.
A swift current occurring in a narrow passage connecting two large bodies of water which is produced by the continuously changing difference in height of tide at the two ends of the passage is called a(n)（）.
A tide is called diurnal when（）.
The presence of stratus clouds and a dying wind will usually result in（）.