$A$ coaxial cable consists of an inner wire of radius $a$ surrounded by an outer shell of inner and outer radii $b$ and $c$ respectively. The inner wire carries an electric current $i_o$,which is distributed uniformly across its cross-sectional area. The outer shell carries an equal current in the opposite direction,also distributed uniformly. What will be the ratio of the magnetic field at a distance $x$ from the axis when $(i)$ $x < a$ and $(ii)$ $a < x < b$?

• A
  $\frac{x^2}{a^2}$
• B
  $\frac{a^2}{x^2}$
• C
  $\frac{x^2}{b^2 - a^2}$
• D
  $\frac{b^2 - a^2}{x^2}$