The $IV$ characteristic of an LED is,

If $a\text{,} b\text{,} c\text{,} d$ are inputs to a gate and $x$ is its output, then, as per the following time graph, the gate is,

Identify the semiconductor devices whose characteristics are given below in the order $\left(1\right)\text{,} \left(2\right)\text{,} \left(3\right)\text{,} \left(4\right)$.

The reading of the ammeter for a silicon diode in the given circuit is,