Two semicircular loops of radii R1 and R2 with a common centre: field at the centre

1. A.
   $\frac{\mu_0 I}{4}\left(\frac{1}{R_1} + \frac{1}{R_2}\right)$
2. B.
   $\frac{\mu_0 l}{4}\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$
3. C.
   $\frac{\mu_0 I}{2\pi}\left(\frac{1}{R_1} + \frac{1}{R_2}\right)$
4. D.
   $\frac{\mu_0 I}{2\pi}\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$

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1. A.
   $\frac{\mu_{0}I}{4}\left\lbrack \frac{1}{R_{1}} - \frac{1}{R_{2}} \right\rbrack$
2. B.
   $\frac{\mu_{0}I}{4}\left\lbrack \frac{1}{R_{2}} - \frac{1}{R_{1}} \right\rbrack$
3. C.
   $\frac{\mu_{0}I}{2\pi}\left\lbrack \frac{1}{R_{1}} - \frac{1}{R_{2}} \right\rbrack$
4. D.
   $\frac{\mu_{0}I}{2\pi}\left\lbrack \frac{1}{R_{2}} - \frac{1}{R_{1}} \right\rbrack$

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