A) iii) Name an application of a combination of a p-type and n-type semiconductor.

The relationship between current ($I$), charge ($Q$), and time ($t$) is given by $Q = I \times t$.

$$Q = 2\, \text{A} \times 4\, \text{s}$$ $$Q = 8\, \text{C}$$

The total charge $Q$ is also equal to the number of electrons ($n$) multiplied by the charge of a single electron ($e$).

$$Q = n \times e$$ $$n = \frac{Q}{e}$$ $$n = \frac{8\, \text{C}}{1.6 \times 10^{-19}\, \text{C}}$$ $$n = 5 \times 10^{19}$$

The number of electrons that flow through the conductor is $\boxed{\text{}5 \times 10^{19}\text{}}$.