Q15. Two conducting circular loops $A$ and $B$ are placed in the same plane with their centers coinciding as shown in\\
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figure. The mutual inductance between them is :\\
(1) $\frac { \mu _ { 0 } \pi b ^ { 2 } } { 2 a }$\\
(2) $\frac { \mu _ { o } } { 2 \pi } \cdot \frac { b ^ { 2 } } { a }$\\
(3) $\frac { \mu _ { 0 } } { 2 \pi } \cdot \frac { a ^ { 2 } } { b }$\\
(4) $\frac { \mu _ { 0 } \pi a ^ { 2 } } { 2 b }$