In a communication system,what is used to extend the range of a communication?

An amplitude modulated wave is represented by $c_m(t)= 10[1+0.6 \sin (1250 t)] \sin (10^8 t)$. Then the modulation index is

Which frequency range is suitable for sky wave propagation?

By what percentage will the transmission range of a $TV$ tower be affected when the height of the tower is increased by $21 \%$ (in $\%$)?

Frequencies in the $UHF$ range normally propagate by means of:

$A$ baseband signal of $3.5\, MHz$ frequency is modulated with a carrier signal of $3.5\, GHz$ frequency using amplitude modulation. What should be the minimum size of antenna required to transmit the modulated signal?