一般在自由空间，距离每增加一倍，声压级将减少（）dB。

A.4

B.5

C.6

D.7

A RADIO SIGNAL FROM A DISTRESSED VESSEL AUTOMATICALLY DIRECTED TO A RCC GIVING POSITION，IDENTIFICATION，COURSE AND SPEED OF THE VESSEL AS WELL AS THE NATURE OF DISTRESS is known as（）.

A.Distress alert

B.Safety alert

C.Emergency alert

D.General alert