Which of the following components acts as a complete electronic circuit?

The frequency response curve of an $RC$ coupled amplifier is shown in the figure. The bandwidth of the amplifier will be

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

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

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

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: