Explanation: Class C amplifiers are biased to conduct current for significantly less than 180 degrees of the input signal cycle. This means they only amplify a small portion of the incoming waveform. When used with complex audio signals, this results in severe clipping and "excessive distortion" because most of the waveform is simply ignored or cut off. Audio signals require faithful reproduction of their entire waveform to sound natural. While Class C is highly efficient and suitable for RF power amplification (where a tuned output circuit can reconstruct a clean sine wave from these brief current pulses), this reconstruction mechanism does not work for the complex, broad-spectrum nature of audio. Therefore, option A is correct. Options B and C are not the primary reasons. While Class C operation involves specific current characteristics and high power can impact tube life, the fundamental issue preventing its use for audio is the inherent waveform distortion.