The magnetic field at the centre of a circular coil of radius $r$ carrying current $I$ is ${B_1}$. The field at the centre of another coil of radius $2 r$ carrying same current $I$ is ${B_2}$. The ratio $\frac{{{B_1}}}{{{B_2}}}$ is
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(c) $B \propto \frac{1}{r} \Rightarrow \frac{{{B_1}}}{{{B_2}}} = \frac{{{r_2}}}{{{r_1}}} = \frac{{2r}}{r} = 2$
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