Two coaxial solenoids $1$ and $2$ of the same length are placed such that one is inside the other. The number of turns per unit length are ${n_1}$ and ${n_2}$. The currents ${i_1}$ and ${i_2}$ are flowing in opposite directions. The magnetic field inside the inner coil is zero. This is possible when:

• A
  ${i_1} \neq {i_2}$ and ${n_1} = {n_2}$
• B
  ${i_1}{n_1} = {i_2}{n_2}$
• C
  ${i_1} = {i_2}$ and ${n_1} = {n_2}$
• D
  Both $(b)$ and $(c)$