Which of the following is true for communication via optical fiber?

For sky wave propagation,the radio waves must have a frequency range in between

The intensity of a light pulse travelling along a communication channel decreases exponentially with distance $x$ according to the relation $I = I_0 e^{-\alpha x}$,where $I_0$ is the intensity at $x = 0$ and $\alpha$ is the attenuation constant. The attenuation in $dB/km$ for an optical fibre in which the intensity falls by $50$ percent over a distance of $50 \ km$ is

If the maximum and minimum amplitudes of a modulated wave are $25 \ V$ and $5 \ V$ respectively,then the modulation index is

If the amplitude of the modulated wave and the amplitude of the signal are equal, then the modulation index is .....$\%$

Which of the following options is $NOT$ correct? We need modulation for: