For short-distance communication,waves with a frequency of $f$ $MHz$ are reflected back from the ionosphere. If the electron density of the ionosphere is $10^{11} \ m^{-3}$,what is the value of $f$?

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

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	$IV$. $65-75 \ km$

$A$ sinusoidal carrier voltage is amplitude modulated. The resultant amplitude modulated wave has maximum and minimum amplitude of $120 \, V$ and $80 \, V$ respectively. The amplitude of each sideband is $.......... \, V$.

$A$ wave is represented as $e = 10 \sin(10^8 t + 6 \sin(1250 t))$. The modulation index is:

For an amplitude modulated wave,the modulation index is found to be $0.5$. If the maximum amplitude is found to be $10.0 \ V$,then the minimum amplitude is: (in $V$)