Field at the centre of a flat spiral coil, or its magnetic moment

1. A.
   $\frac{\mu_{0}IN}{2(b - a)}\log_{e}\left( \frac{b}{a} \right)$
2. B.
   $\frac{\mu_{0}I}{8}\left\lbrack \frac{a + b}{a - b} \right\rbrack$
3. C.
   $\frac{\mu_{0}I}{4(a - b)}\left\lbrack \frac{1}{a} - \frac{1}{b} \right\rbrack$
4. D.
   $\frac{\mu_{0}I}{8}\left( \frac{a - b}{a + b} \right)$

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1. A.
   4.4
2. B.
   2.64
3. C.
   3.25
4. D.
   1.2

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