Range of frequencies allotted for commercial $FM$ radio broadcast is

In amplitude modulation,if a message signal of $5 \ kHz$ is modulated by a carrier wave of frequency $900 \ kHz$,then the frequencies of the side bands are

$AM$ is used for broadcasting because...

Long range radio transmission is possible when the radio waves are reflected from the ionosphere. For this to happen,the frequency of the radio waves must be in the range:

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 $\,km$)

The heights of transmitting and receiving antennas are respectively $\frac{1}{20000}$ and $\frac{1}{80000}$ times the radius of the earth. The maximum distance between these two antennas for satisfactory communication in line of sight mode is (Radius of the earth $= 6.4 \times 10^6 \ m$) (in $km$)