The signal obtained at the output of a microphone is of which type?

$A$ signal of frequency $3 \text{ kHz}$ is amplitude modulated on a carrier wave of frequency $2.5 \text{ MHz}$. The upper and lower sideband frequencies in the resultant signal are

$AM$ (Amplitude Modulation) is used for broadcasting because...

The digital signals are used for transmission in communication systems. They are

We do not transmit low frequency signals to long distances because:
$(a)$ The size of the antenna should be comparable to signal wavelength,which is impractical for a signal of longer wavelength.
$(b)$ Effective power radiated by a long wavelength baseband signal would be high.
$(c)$ We want to avoid mixing up signals transmitted by different transmitters simultaneously.
$(d)$ Low frequency signals can be sent to long distances by superimposing them with a high frequency wave.
Therefore,the most suitable options are:

Match List-$I$ with List-$II$:

List-$I$	List-$II$
$A$. Troposphere	$I$. Approximate $65-75 \ km$ over Earth's surface
$B$. $E$-Part of Ionosphere	$II$. Approximate $300 \ km$ over Earth's surface
$C$. $F_2$-Part of Ionosphere	$III$. Approximate $10 \ km$ over Earth's surface
$D$. $D$-Part of Ionosphere	$IV$. Approximate $100 \ km$ over Earth's surface

Choose the correct answer from the options given below: