$AM$ is used for broadcasting because...

An oscillator is producing $FM$ waves of frequency $2 \text{ kHz}$ with a frequency variation of $10 \text{ kHz}$. What is the modulation index?

This question has Statement-$1$ and Statement-$2$. Of the four choices given after the statements,choose the one that best describes the two statements.
Statement-$1$: Sky wave signals are used for long-distance radio communication. These signals are,in general,less stable than ground wave signals.
Statement-$2$: The state of the ionosphere varies from hour to hour,day to day,and season to season.

If the ratio of the maximum and minimum amplitudes of an amplitude modulated wave is $7:3$,then the modulation index is

Audio signals cannot be transmitted directly because $.......$

The electron density of the ionospheric layer is $10^{11} \ m^{-3}$. Communication waves of frequency up to $f_c$ $MHz$ will be reflected by the ionospheric layer. Find $f_c$.