Jake and Bill Are Both College Graduates

Jake and Bill are both college graduates.Jake is a patrolman and Bill is a detective in the same police precinct.While Jake's job is inherently more dangerous than Bill's,Bill passed a difficult exam to gain promotion to detective.Bill earns more than Jake because

A) of a compensating differential.
B) of efficiency wages.
C) of education as a signal.
D) Bill has more human capital.