Indicate which one of the following systems is digital.

For transmitting a signal of frequency $1 \text{ kHz}$,the wavelength of the wave should be:

$A$ carrier wave $v_{c}(t) = 160 \sin(2 \pi \times 10^{6} t) \text{ V}$ is made to vary between $V_{\max} = 200 \text{ V}$ and $V_{\min} = 120 \text{ V}$ by a message signal $v_{m}(t) = A_{m} \sin(2 \pi \times 10^{3} t) \text{ V}$. The peak voltage $A_{m}$ of the modulating signal is ..... .

The audio signal used to modulate $60 \sin(2\pi \times 10^6 t)$ is $15 \sin(300\pi t)$. The depth of modulation is.......$\%$

Television signals on earth cannot be received at distances greater than $100\, km$ from the transmission station. The reason behind this is that

For the propagation of $10 \ MHz$ signals as sky waves,what should be the minimum electron density in the ionosphere?