Explanation: A vacuum tube operating as a Class A amplifier is biased such that plate current flows for the entire 360 degrees of the input signal cycle. This ensures the output waveform is a faithful, undistorted reproduction of the input, making it ideal for high-fidelity audio applications. Due to the constant current flow, even with no input signal, Class A amplifiers have very low plate circuit efficiency, typically around 20-30% (25% is a good approximation). Additionally, the grid is usually biased to remain negative, preventing grid current and thus requiring practically no grid driving power. Option B describes characteristics more typical of a Class AB amplifier, where current flows for slightly more than 180 degrees and efficiency is higher. Option C describes a Class C amplifier, known for high efficiency but significant distortion due to current flowing for less than 180 degrees (in "bursts"). Therefore, option A accurately summarizes the defining features of a Class A vacuum tube amplifier.