When a potential difference is applied across,the current passing through:

The time variations of signals $A, B$ and $C$ are shown below. Choose the correct statement.

The negative feedback in an amplifier

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:

Identify the semiconductor devices whose characteristics are given below,in the order $(a), (b), (c), (d)$.

Given below are two statements:
Statement $I$: Photovoltaic devices can convert optical radiation into electricity.
Statement $II$: Zener diode is designed to operate under reverse bias in the breakdown region.
In the light of the above statements,choose the most appropriate answer from the options given below: