The process of superimposing a message signal on a high-frequency carrier wave is called:

The frequency of a $FM$ transmitter without signal input is called

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	$IV$. $65-75 \ km$

$A$ receiving station on the ground is receiving a signal of frequency $5 \ MHz$ from a transmitter. Then the signal can come via

$A$ carrier signal $C(t)=25 \sin(2.512 \times 10^{10} t)$ is amplitude modulated by a message signal $m(t)=5 \sin(1.57 \times 10^{8} t)$ and transmitted through an antenna. What will be the bandwidth of the modulated signal?

The transmitting antenna placed at the top of a tower has a height of $45 \ m$ from the ground. The distance between receiving and transmitting antennas is $40 \ km$ and the radius of earth is $6400 \ km$. The minimum height (in $m$) at which the receiving antenna is to be placed for satisfactory communication in $LOS$ mode, is