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In the figure shown there are two semicircles of radii r1 and r2 in which a current i is flowing. The magnetic induction at the centre O will be
(a) μ0irr1+r2 (c) μ0i4r1+r2r1r2
(b) μ0i4(r1-r2) (d) μ0i4r2-r1r1r2