For sky wave propagation,the radio waves must have a frequency range in between

Assertion $(A)$: Television signals are received through sky-wave propagation.
Reason $(R)$: The ionosphere reflects electromagnetic waves of frequencies in the range $(3-30)$ $MHz$.
Choose the correct option.

Consider telecommunication through optical fibres. Which of the following statements is $NOT$ true?

$A$ carrier wave of amplitude $15\,V$ is modulated by a sinusoidal baseband signal of amplitude $3\,V$. The ratio of maximum amplitude to minimum amplitude in an amplitude modulated wave is:

What is the minimum height of an antenna required to transmit a radio signal of frequency $20 \text{ MHz}$?

The radio horizon of the transmission of an antenna placed on the $20^{\text{th}}$ floor in a shopping mall,where the height of each floor is $2 \,m$,is (Radius of Earth is $6.4 \times 10^6 \,m$): (in $\,km$)