The plates of a parallel capacitor are charged up to $100 \mathrm{~V}$. If $2 \mathrm{~mm}$ thick plate is inserted between the plates, then to maintain the same potential difference, the distance between the capacitor plates is increased by $1.6 \mathrm{~mm}$ the dielectric constant of the plate is
(A) 5
(B) 4
(C) $1.25$
(D) $2.5$