Marina Exam for OIC: Which signal may be sounded ONLY by vessels in restricted visibility?

Marina Exam for OIC

Which signal may be sounded ONLY by vessels in restricted visibility? 

A. At least five short and rapid blasts 
B. Four short blasts 
C. Two short blasts 
D. One prolonged, one short, one prolonged, and one short blast, in that order 


Answer:  B.  Four short blasts