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