Explanation: Class B amplifiers are designed to conduct current for approximately 180 degrees (half) of the input signal cycle. This operating point offers a significant efficiency improvement over Class A amplifiers, which conduct for the full 360 degrees and typically achieve efficiencies in the 20-30% range (option A). While the theoretical maximum efficiency for a Class B amplifier is around 78.5%, practical vacuum tube circuits always have losses due to component resistance, drive power requirements, and non-ideal operating conditions. These factors bring the typical achievable efficiency down to approximately 60%. Options C (85%) and D (100%) are generally too high for practical Class B operation; 100% efficiency is thermodynamically impossible for any real-world amplifier.