A wire of circular cross section with radius $a$ is shown in the figure. The current density is given by $\mathbf{J}=k s^{2}$, where $k$ is a constant, $s$ is the radial distance from the axis and $0 \leq s \leq a$. The total current $I$ in the wire is
$\frac{\pi k a^{4}}{2}$
$\frac{2 \pi k a^{3}}{3}$
$\frac{\pi k a^{3}}{2}$
$\frac{\pi k a^{4}}{4}$