A.at least annually B.whenever it is below its required weight C.only if the extinguisher has been used D.before every safety inspection
单项选择题The emergency signal for fires is sounded on the ship’s whistle and general alarm as（）.
A.a continuous ringing for 10 seconds B.one short ring followed by one long ring C.two long rings of at least 20 seconds D.a continuous ringing until the fire is extinguished
