The ionosphere acts as a reflector for which frequency range?

In a communication system,a repeater is used to extend the range of transmission. It is a combination of:

If in an amplitude modulated wave,the maximum amplitude is $14 \ V$ and the modulation index is $0.4$,then the amplitude of the carrier wave is (in $V$)

In amplitude modulation,the modulation index $m$ is kept less than or equal to $1$ because

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

The amplitude of $15 \sin (1000 \pi t)$ is modulated by $10 \sin (4 \pi t)$ signal. The amplitude modulated signal contains frequency(ies) of
$A) 500 \text{ Hz}$
$B) 2 \text{ Hz}$
$C) 250 \text{ Hz}$
$D) 498 \text{ Hz}$
$E) 502 \text{ Hz}$
Choose the correct answer from the options given below: