The cross$-$sectional area of an adjustable single loop is reduced from $1\mathrm{.}0m^2$ to $0\mathrm{.}25m^2$ in $0\mathrm{.}10$ s $.$ What is the average emf that is induced in this coil if it is in a region where $B=2\mathrm{.}0T$ upward, and the coil's plane is perpendicular to $B?$

$\frac{?}{{?}_{\frac{?}{?}}A}=0\mathrm{.}75m^2,B=2\mathrm{.}07$

D$)12$ V

E$)21$ V

A$)0$ V

B$)18$ V

$t=0\mathrm{.}10k$

C$)15$ V

D$)12$ V