$A$ coaxial cable having radii $a, b$ and $c$ carries equal and opposite currents of magnitude $i$ in the inner and outer conductors. What is the magnitude of the magnetic induction at point $P$ outside of the cable at a distance $r$ from the axis?

• A
  $0$
• B
  $\frac{\mu_0 i r}{2 \pi a^2}$
• C
  $\frac{\mu_0 i}{2 \pi r}$
• D
  $\frac{\mu_0 i}{2 \pi r} \frac{c^2 - r^2}{c^2 - b^2}$