单项选择题
Clouds form（）.
A.as a mass of warm，humid air rises into the atmosphere and cools，condensing moisture into small droplets
B.as winds blow across bodies of water，the sun causes the moisture to be absorbed and move upward forming clouds
C.dry air compresses moisture from the atmosphere into clouds
D.when the relative humidity of the atmosphere is low
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A.high pressure fluid accumulators
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C.filling open-circuit breathing systems
D.bladder-type accumulators -
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Chronometer error may be found by（）.
A.radio time signal
B.comparison with a timepiece of known error
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D.All of the above
