Explanation: Tetrode vacuum tubes exhibit very high plate resistance ($R_p$). For efficient power transfer and optimal performance in an audio amplifier, the load impedance presented to the plate circuit by the output transformer's primary winding should be significantly higher than the tube's internal plate resistance. An ordinary audio transformer, not specifically designed for tetrodes or pentodes, typically provides a primary impedance that is too low to effectively match the very high plate resistance of a tetrode. This impedance mismatch would lead to poor power transfer, reduced voltage gain, and inefficient operation of the stage. Therefore, specialized output transformers with much higher primary impedances are required for tetrode and pentode amplifiers. Option B is incorrect because too *much* impedance would actually be *closer* to what a tetrode needs, though specific matching is still critical. Option C, cost, is not the fundamental electrical reason for the design limitation.