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

The ratio of the maximum amplitude to the minimum amplitude of an amplitude-modulated wave is $4$. What is the modulation index of this wave?

If $E_c$ and $E_m$ are peak values of carrier and modulating signals,respectively,then for $100 \%$ modulation,

The maximum distance up to which $TV$ transmission from a $TV$ tower of height $h$ can be received is proportional to

If the maximum and minimum amplitudes of a modulated wave are $25 \ V$ and $5 \ V$ respectively,then the modulation index is

Match Column $A$ (layers in the ionosphere for skywave propagation) with Column $B$ (their height range).

Column $A$	Column $B$
$A$. $D$-layer	$I$. $250-400 \ km$
$B$. $E$-layer	$II$. $170-190 \ km$
$C$. $F_1$-layer	$III$. $95-120 \ km$
$D$. $F_2$-layer	$IV$. $65-75 \ km$