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、A.Their bearing is almost constant，even when the true bearing of the contact changes appreciably

B、B.They always appear on a bearing of 90°From the true bearing of the contact

C、C.The indirect echoes usually appear in shadow sectors

D、D.When plotted，their movements are usually abnormal