Two coils are kept near each other. When no current passes through the first coil and the current in the $2^{nd}$ coil increases at the rate of $10 \,A/s$, the e.m.f. in the $1^{st}$ coil is $20 \,mV$. When no current passes through the $2^{nd}$ coil and $3.6 \,A$ current passes through the $1^{st}$ coil, the flux linkage in coil $2$ is:

• A
  $1.2 \times 10^{-3} \,Wb$
• B
  $1.8 \times 10^{-3} \,Wb$
• C
  $3.6 \times 10^{-3} \,Wb$
• D
  $7.2 \times 10^{-3} \,Wb$