# The current in the primary circuit of a potentiometer is $0.2 A$. The specific resistance and cross-section of the potentiometer wire are $4 \times 10^{-7} ohm$ meter and $8 \times 10^{-7} \: m^2$ respectively. The potential gradient will be equal to:
$\begin {array} {1 1} (A)\;1 V/m & \quad (B)\;0.5 V/m \\ (C)\;0.1 V/m & \quad (D)\;0.25 V/m \end {array}$