$A$ coil having $N$ turns is wound tightly in the form of a spiral with inner and outer radii $a$ and $b$ respectively. When a current $i$ passes through the coil,the magnetic field at the centre is

  • A
    $\frac{\mu_0 Ni}{b}$
  • B
    $\frac{2\mu_0 Ni}{a}$
  • C
    $\frac{\mu_0 Ni}{2(b - a)} \ln\left(\frac{b}{a}\right)$
  • D
    $\frac{\mu_0 Ni}{(b - a)} \ln\left(\frac{b}{a}\right)$

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