An audio signal $v_{m} = 20 \sin(2 \pi \times 1500 t)$ amplitude modulates a carrier $v_{c} = 80 \sin(2 \pi \times 100,000 t)$. The value of percent modulation is..........

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

List-$I$ (Layer of atmosphere)	List-$II$ (Approximate height over earth's surface)
$(A)$ $F_1$-Layer	$(I)$ $10\,km$
$(B)$ $D$-Layer	$(II)$ $170-190\,km$
$(C)$ Troposphere	$(III)$ $100\,km$
$(D)$ $E$-Layer	$(IV)$ $65-75\,km$

Choose the correct answer from the options given below:

The modulation index for an $A.M.$ wave having maximum and minimum peak-to-peak voltages of $14\,mV$ and $6\,mV$ respectively is:

Modulation is a process of superposing

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

The electron density of $E, F_1, F_2$ layers of the ionosphere is $2 \times 10^{11} \, m^{-3}$,$5 \times 10^{11} \, m^{-3}$,and $8 \times 10^{11} \, m^{-3}$ respectively. What is the ratio of critical frequencies for the reflection of radio waves?