Explanation: Harmonic radiation consists of signals emitted at frequencies that are whole number multiples (e.g., 2x, 3x, 4x) of the intended operating frequency, known as the fundamental frequency. Therefore, statement A is correct. Since these harmonic frequencies are multiples of the fundamental, they will always be higher in frequency than the fundamental itself, making statement B correct. To comply with regulations and prevent interference, amateur radio operators must minimize harmonic emissions. Proper tuning of the transmitter's output stage, along with the use of external low-pass filters, effectively attenuates these unwanted harmonics. This makes statement C correct. As all three statements accurately describe aspects of harmonic radiation, "All of the above" is the correct answer.