单项选择题
A.When the distress alert is sounded
B.When the emergency equipment is switching on
C.When the general emergency alarm is sounded
D.When the distress broadcast is sounded
单项选择题 We have just received a message from shore（）assistance is on the way.
单项选择题 （）we stand away from the church bell, it takes time for the sound waves to reach us.
单项选择题 Before（）, a station shall take precautions to ensure that its emission will not interfere with transmission already in progress.