AREAS WHICH CANNOT BE SCANNED BY THE RADAR OF THE VESSEL BECAUSE THEY ARE SHIELDED BY PARTS OF ITS SUPERSTRUCTURE，MASTS，ETC means（）.

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

A.radio signals reflect from a solid surface
B.sound waves travel at a constant speed through water
C.radar signals travel at a constant speed through water
D.pressure increases with depth

A.Water temperature
B.Climatology data
C.Speed of advance
D.Depth contours