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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