问题:
A . a 30 second on 30 second off alternating signal
B . the continuous blast of the ships whistle for not less than 10 seconds supplemented by the continuous ringing of the general alarm bells for not less than 10 seconds
C . an intermittent ringing of the general alarm for not less than ten seconds
D . announced over the PA system
● 参考解析
本题暂无解析