Explanation: Class B amplifiers are biased so that each active device (tube in this case) conducts for approximately 180 degrees of the input signal cycle. If only one tube were used, it would amplify only half of the input waveform (either the positive or negative half), completely cutting off the other half. This would result in severe, unacceptable distortion, as the output would be a highly truncated version of the input signal. To overcome this, two tubes are used in a "push-pull" configuration. One tube is configured to amplify the positive half of the input signal, while the other amplifies the negative half. Their outputs are then combined, reconstructing a complete, albeit still somewhat distorted due to crossover effects, approximation of the original waveform. This prevents the gross distortion that a single Class B tube would produce. Option B is incorrect because while a balanced push-pull configuration can offer some rejection of common-mode noise like hum, the fundamental reason for using two tubes in Class B is to amplify both halves of the signal and prevent waveform mutilation, not primarily to eliminate hum. Therefore, C is also incorrect.