(Solved): As in the figure, the long cylindrical slice with inner radius a and outer radius 2a has a current ...

As in the figure, the long cylindrical slice with inner radius $a$ and outer radius $2a$ has a current density given by $J=J_0\frac{r}{a}$. Where $r$ is the radial distance from the axis of the cylinder. If the current flowing in the $z$ direction is $I=7(A)$ and $a=2(m)$, what is the constant $J_0$ in $\frac{A}{m^2}?(?=3)$ A) $\frac{1}{28}$ ?) $\frac{1}{8}$ C) $\frac{1}{7}$ D) $\frac{1}{21}$ E) $\frac{3}{7}$