A conducting metal circular-wire-loop of radius $r$ is placed perpendicular to a magnetic field which varies with time as $B = B_0 e^{-t/τ} $= , where $B_0$ and $τ$ are constants, at time $t=0$. If the resistance of the loop is $R$ then the heat generated in the loop after a long time $( t →\propto)$ is :