The correct answer is B: Some modes have high duty cycles that could exceed the transmitter's average power rating. Why it is important to know the duty cycle of the mode you are using when transmitting is that some modes have high duty cycles that could exceed the transmitter's average power rating. High duty cycle modes (like RTTY, some digital modes) transmit continuously, which can overheat transmitters rated for lower duty cycles. For amateur radio operators, this is critical for transmitter protection. Understanding this helps prevent equipment damage.