$A$ 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
    $\frac{{\mu _0}NI}{{2(b - a)}}\ln \frac{b}{a}$
  • D
    $\frac{{\mu _0}{I^N}}{{2(b - a)}}\ln \frac{b}{a}$

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