有人看守道口小心火车牌标志：设在通向道路的右侧，距离铁路道口最外股钢轨不少于20米处，为黑边黄底黑色蒸汽机图案的三角形警告标。
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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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