根据《1979年国际海上搜寻救助公约》，"Alert phase" is()
（A）A generic term meaning, as the case may be,uncertainty phase, alert phase or distress phase.
（B）A situation wherein uncertainty exists as to the safety of a vessel and the persons on board.
（C）A situation wherein apprehension exists as to the safety of a vessel and of the persons on board.
（D）A situation wherein there is a reasonable certainty that a vessel or a person is threatened by grave and imminent danger and requires immediate assistance.
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