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

If a $TV$ transmission is broadcast from a $TV$ transmitter tower of height $h$,then the maximum distance of this transmission is proportional to the height $h$ of the tower as.........

If a receiving station on the ground is receiving a frequency of $10 \text{ MHz}$, the mode of propagation is...

$A$ radar has a power of $1 \ kW$ and is operating at a frequency of $10 \ GHz$. It is located on a mountain top of height $500 \ m$. The maximum distance up to which it can detect an object located on the surface of the earth (Radius of earth $= 6.4 \times 10^6 \ m$) is ....... $km$.

$A$ signal of $5\ kHz$ frequency is amplitude modulated on a carrier wave of frequency $2\ MHz$. The frequencies of the resultant signal is/are

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.