A parallel plate capacitor having cross-sectional area A and separation d has air in between the plates. Now an insulating slab of same area but thickness d/2 is inserted between the plates as shown in figure having dielectric constant K(= 4). The ratio of new capacitance to its original capacitance will be :
(1) 4 : 1
(2) 2 : 1
(3) 8 : 5
(4) 6 : 5