Potentiometer wire is of cross sectional area $8 \times 10^{-6}m^2$ and has resistivity $40 \times 10^{-8}$. If $0.2$ amp current flows through it its potential gradient is

Question

$(a)\;10^{-2} v/m \\(b)\;2 \times 10^{-2}\;v/m \\(c)\;10^{-3} \;v/m \\(d)\;3 \times 10^{-3}\;v/m $