Explanation: Using an AC filament supply is primarily desirable for practical reasons. Alternating current (AC) is readily available from standard mains power outlets, and stepping it down to the required voltage for tube filaments only requires a simple and inexpensive transformer. This makes the design and construction of the power supply much simpler and more cost-effective compared to generating a filtered and regulated DC supply for the filaments. Option B is incorrect because hum being introduced into the amplifier is a *potential disadvantage* of using AC filaments, not a desirable characteristic. While methods like center-tapping the filament winding to ground can minimize hum, the possibility of hum is a trade-off, not a reason for desirability. Therefore, stating that hum *may be introduced* doesn't explain why AC filaments are *desirable*.