Which of the following devices is not a transducer?

$A$ message signal of frequency $10 kHz$ is used to modulate a carrier wave of frequency $6 MHz$. The sideband frequencies are:

When the modulating frequency is doubled, the modulation index is halved and the modulating voltage remains constant, the modulation system is

$A$ $TV$ tower has a height of $150 \ m$. If the population density around the $TV$ tower is $10^3 \ km^{-2}$, then the population covered by the tower is (Radius of the earth, $R = 6.4 \times 10^6 \ m$). (in $\text{lakh}$)

Which of the following options is $NOT$ correct? We need modulation for:

In the process of modulation,........