Why Amplifier class is set by how much of the input cycle the device conducts, and less conduction time means less time dissipating heat. Class A conducts the full 360 degrees (about 25-30% efficient), Class B about 180 degrees (up to ~60%), Class AB somewhere between, and Class C conducts for less than 180 degrees, giving efficiencies of roughly 70-80%. The tradeoff is distortion: Class C is very nonlinear, so it is used for CW, FM, and frequency multipliers where a tuned output circuit restores the waveform, not for SSB.
Watch out Class A is the tempting pick because it is the best amplifier in terms of linearity and low distortion, but it is the worst in efficiency since the device draws current the entire cycle, even with no signal.
Less conduction angle = more efficiency: A (360) < AB < B (180) < C (under 180). C is the efficiency champ, linearity loser.