Explanation: To determine the cathode-bias resistance, one utilizes the voltage drop across a resistor placed in the cathode circuit. This voltage drop, an "IR drop" per Ohm's Law, constitutes the negative grid bias relative to the cathode (A). The resistance value is calculated using Ohm's Law: resistance equals the desired DC bias voltage divided by the total no-signal current flowing through the cathode (B). For tubes with multiple grids, specifically tetrodes and pentodes, the total current flowing through the cathode resistor includes not only the plate current but also the screen grid current, as both paths flow from the cathode to ground (C). Therefore, all three statements are essential principles and steps for correctly determining the necessary cathode-bias resistance for an amplifier.