单项选择题
A.Their bearing is almost constant，even when the true bearing of the contact changes appreciably
B.They always appear on a bearing of 90°From the true bearing of the contact
C.The indirect echoes usually appear in shadow sectors
D.When plotted，their movements are usually abnormal
单项选择题 An icebreaker may use the code letter “K” to remind ships of their obligation to listen continuously on their radio. This signal may be made by any of the following EXCEPT（）.
单项选择题 An icebreaker displaying a flag hoist which consists of the code letters “WO” (Whiskey Oscar) is indicating that（）.
单项选择题 An icebreaker assisting a vessel through an ice field would display a visual signal consisting of the code letter “E” (Echo) to signify that “（）”.