An otherwise infinite, straight wire has two concentric loops of radii a and b carrying equal
currents in opposite directions as shown. The magnetic field at the common centre is zero for

\[\frac{a}{b}=\frac{\pi+1}{\pi}\]
\[\frac{a}{b}=\frac{\pi}{\pi+1}\]
\[\frac{a}{b}=\frac{\pi-1}{\pi+1}\]
\[\frac{a}{b}=\frac{\pi+1}{\pi-1}\]
No solution provided for this question.
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