Infinitely long wire bent to form a circular loop: field at the centre

1. A.
   Zero
2. B.
   $\frac{\mu_0}{4\pi}\frac{2}{r}(1+\pi)$
3. C.
   $\frac{\mu_0}{4\pi}\frac{I}{r}(1+\pi)$
4. D.
   $\frac{\mu_0}{4\pi}\frac{2I}{r}(\pi-1)$

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1. A.
   zero
2. B.
   $\frac{\mu_{0}I}{4\pi r}(\pi - 1)$
3. C.
   $\frac{\mu_{0}I}{2\pi r}(\pi + 1)$
4. D.
   $\frac{\mu_{0}I}{2\pi r}(\pi - 1)$

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