An audio signal of $15\;KHz$ , frequency can not be transmitted over long distances without modulation because

Question

$\begin{array}{1 1} \text{The size of the required antenna would be at least 5 km which is not convenient } \\ \text{The audio signal cannot be transmitted through sky waves} \\ \text{The size of the required antenna would be at least 20 km which is not convenient } \\ \text{effective power of transmitted wold be very low, if the size of the antenna is less than 5 m } \end{array}$