Marina Exam for OIC: Which signal is sounded ONLY by a vessel in sight of another?

Marina Exam for OIC 

Which signal is sounded ONLY by a vessel in sight of another? 

A. One short blast on the whistle 
B. Four short blasts on the whistle 
C. One short, one prolonged, and one short blast on the whistle 
D. One prolonged blast on the whistle 


Answer:  A. One short blast on the whistle