单项选择题
An indirect radar echo is caused by a reflection of the main lobe of the radar beam off the observer’s vessel. Which of the following is NOT a characteristic of indirect echoes? （）
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
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