The wire loop $PQRSP$ formed by joining two semicircular wires of radii $R_1$ and $R_2$ carries a current $I$ as shown. The magnitude of the magnetic field at the centre '$O$' is

  • A
    $\frac{\mu_0 I}{4}\left[\frac{1}{R_1}-\frac{1}{R_2}\right]$
  • B
    $\frac{\mu_0 I}{4}\left[\frac{1}{R_2}-\frac{1}{R_1}\right]$
  • C
    $\frac{\mu_0 I}{2 \pi}\left[\frac{1}{R_1}-\frac{1}{R_2}\right]$
  • D
    $\frac{\mu_0 I}{2 \pi}\left[\frac{1}{R_2}-\frac{1}{R_1}\right]$

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