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, (2020-Covid)
a. 8 : 5
b. 6 : 5
c. 4 : 1
d. 2 : 1