Marina Exam Reviewer: Which signal is required to be sounded by a power-driven vessel ONLY?

Marina Exam Reviewer

Which signal is required to be sounded by a power-driven vessel ONLY? 

A. A signal meaning that the vessel sounding it is in doubt as to the other vessel's actions. 
B. A signal meaning, "I intend to overtake you on your starboard side." 
C. A signal meaning, "I am altering my course to starboard." 
D. A signal sounded when approaching a bend. 

Answer:  C.  A signal meaning, "I am altering my course to starboard."