Explanation: Over-excitation of a Class B amplifier's grid circuit means the input signal amplitude is too high, pushing the amplifier beyond its intended linear operating range. Firstly, when the grid is driven too far positive, it can become forward-biased with respect to the cathode, drawing **excessive grid current**. This loads the preceding driver stage and can cause signal clipping. Secondly, operating outside the linear region introduces significant non-linearity, resulting in **excessive distortion** of the output signal. The waveform is no longer a faithful reproduction of the input, generating unwanted harmonics and intermodulation products. Thirdly, this non-linear operation, often leading to saturation or severe clipping, causes the tube to operate inefficiently, leading to **decreased plate efficiency**. More DC power is dissipated as heat within the tube rather than converted to useful RF output. Furthermore, driving the grid excessively positive can also lead to **excessive plate current**, potentially damaging the tube or power supply. Therefore, both B and C are correct, making D the best answer.