Explanation: A bypass capacitor across a cathode-bias resistor in an RF amplifier serves two critical functions. First, (A) it prevents variations of the DC bias during excitation. The cathode resistor establishes a stable DC bias voltage for the tube or transistor. When an AC signal excites the amplifier, the current through the active device fluctuates. Without the bypass capacitor, these AC current variations would flow through the cathode resistor, developing an AC voltage across it that opposes the input signal. This effectively makes the bias "wobble" with the signal, reducing bias stability. The capacitor provides a low-impedance path for these AC currents, shunting them around the resistor and maintaining a steady DC bias. Second, (B) this action results in better gain for the amplifier stage. The AC voltage developed across an unbypassed cathode resistor acts as degenerative feedback, effectively reducing the input signal seen by the amplifier and thus lowering its overall gain. By providing a low-impedance path for the AC signal current, the bypass capacitor eliminates this negative feedback for AC signals, allowing the amplifier to achieve its maximum possible gain. Therefore, both A and B are correct.