$A$ 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
    $\frac{\mu_0 IN}{(b - a)} \ln \left( \frac{b}{a} \right)$

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