根据《1979年国际海上搜寻救助公约》，"Uncertainty phase" is()

AA generic term meaning, as the case may be,uncertainty phase, alert phase or distress phase.

BA situation where in uncertainty exists as to the safety of a vessel and the persons on board.

CA situation wherein apprehension exists as to the safety of a vessel and of the persons on board.

DA situation wherein there is a reasonable certainty that a vessel or a person is threatened by grave and imminent danger and requires immediate assistance.