$A$ current carrying circular loop of radius '$R$' and a current carrying long straight wire are placed in the same plane. $I_c$ and $I_w$ are the currents through the circular loop and the long straight wire,respectively. The perpendicular distance between the centre of the circular loop and the wire is '$d$'. The magnetic field at the centre of the loop will be zero when the separation '$d$' is equal to:

• A
  $\frac{R I_w}{\pi I_c}$
• B
  $\frac{R I_c}{\pi I_w}$
• C
  $\frac{\pi I_c}{R I_w}$
• D
  $\frac{\pi I_w}{R I_c}$