Why is fuse wire always placed in the live wire of an electric circuit?

Two fuse wires A and B of the same length are rated $15 \mathrm{A}$ and $5 \mathrm{A}$. Which amongst the A and B will be thicker and why?

A cell, a resistor, a key, and ammeter are arranged as shown in the diagrams. The current recorded in the ammeter will be:

                         (i)

                         (ii)

                         (iii) 

Identify the circuit in which the electrical components have been properly connected.