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

In a communication system,the process of superimposing a low-frequency signal on a high-frequency wave is known as:

This question has Statement-$1$ and Statement-$2$. Of the four choices given after the statements,choose the one that best describes the two statements.
Statement-$1$: Sky wave signals are used for long-distance radio communication. These signals are,in general,less stable than ground wave signals.
Statement-$2$: The state of the ionosphere varies from hour to hour,day to day,and season to season.

$A$ carrier wave of peak voltage $12 \,V$ is used to transmit a signal. If the modulation index is $75 \%$, the peak voltage of the modulating signal is (in $\,V$)

The transmission loss per kilometer in an optical fiber is approximately...........$dB$.

The bandwidth of a coaxial cable is approximately ........ .