An audio signal of 15 kHz frequency cannot be transmitted over long distances without modulation, because
(a) the size of the required antenna would be at least 5 km which is not convenient
(b) the audio signal cannot be transmitted through sky waves
(c) the size of the required antenna would be at least 20 km, which is not convenient
(d) effective power transmitted would be very low, if the size of the antenna is less than 5 km
The correct answer is the options (a, b, d)
The audio frequency signals transmitted are of extremely low-frequency waves, and due to the absorption of sky waves by the atmosphere, they cannot transmit the radio waves. And the size of the antenna used must not be less than 5 km as it would degrade the quality of transmission significantly. This would be due to the resonance of wavelength of wave and antenna length. So, the correct options are (a), (b), and (d).