Which of the following is used to produce radio waves of constant amplitude?

The $I-V$ characteristic of an $LED$ is:

An amplifier has a voltage gain $A_v = 1000$. The voltage gain in $dB$ is...........

Arvind needs to identify the electrical components from a group of components labelled as $A, B, C$ and $D$. He performed some simple tests and obtained the following observations:
$(a)$ $A$ has a silver ring at one end,with two terminals. It conducts electricity only in one direction.
$(b)$ $B$ is connected to a $D.C.$ source,but the current gradually decreases and becomes zero. It is a two-terminal device and behaves the same in both directions.
$(c)$ $C$ is a two-terminal device,which conducts electricity only in one direction,while emitting light.
$(d)$ $D$ is a two-terminal device with several coloured rings on it. It conducts current in both directions with the same steady current.
The correct identification of $A, B, C$ and $D$ are respectively:

Find the value of current $I$ in the following circuits.

Three amplifiers with voltage gains $10, 20$ and $30$ are connected in series. If the input signal has a peak value of $1 mV$, then the peak value of the output voltage is (in $\,V$)