Which frequency range is suitable for sky wave propagation?

In frequency modulation,........

$A$ speech signal of $3 kHz$ is used to modulate a carrier signal of frequency $1 MHz$,using amplitude modulation. The frequencies of the side bands will be

For efficient transmission of sound waves of wavelength $\lambda$,the minimum length of the antenna should be.....

The modulation frequency of an $AM$ radio station is $250\, kHz$,which is $10\%$ of the carrier wave. If another $AM$ station approaches you for license,what broadcast frequency will you allot? (Note: The interval between two carrier frequencies should be at least two times the modulation frequency to avoid interference.)

Two identical antennas mounted on identical towers are separated from each other by a distance of $45 \, km$. What should be the minimum height of the receiving antenna to receive the signals in line of sight? (Assume radius of earth is $6400 \, km$)