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In the figure shown there are two semicircles of radii ${r_1}$ and ${r_2}$ in which a current $i$ is flowing. The magnetic induction at the centre $O$ will be
  • A$\frac{{{\mu _0}i}}{r}({r_1} + {r_2})$
  • B$\frac{{{\mu _0}i}}{4}({r_1} - {r_2})$
  • C$\frac{{{\mu _0}i}}{4}\left( {\frac{{{r_1} + {r_2}}}{{{r_1}{r_2}}}} \right)$
  • D$\frac{{{\mu _0}i}}{4}\left( {\frac{{{r_2} - {r_1}}}{{{r_1}{r_2}}}} \right)$
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