A current $I= 10 \sin (100 \pi t)$ ampere is passed in first coil which induces a maximum emf of $5 \pi$ volts in second coil. The mutual inductance M is

Question

$(a)\;10\;mH \\ (b)\;5\;mH \\(c)\;25\;mH \\(d)\;15\;mH$