A parallel plate capacitor c with plates of unit area and separation d is filled with a liquid of dielectric constant $k=2$ The level of the liquid is$\large\frac{d}{3}$ initially . Suppose the liquid level decreases at constant speed V, the time constant as a function of time t is

Question

$\begin{array}{1 1} \frac{6 \in_0 R}{5d+3vt} \\ \frac{6 \in_0 R}{5d-3vt} \\ 0 \\ none\; of\; the\; above \end{array} $