A parallel-plate capacitor of vacuum capacitance C0 has plate separation d. A dielectric slab of constant κ=4 and thickness 2d is inserted so that it completely fills the lower half of the space between the plates, while the upper half remains vacuum. The effective capacitance of the system is
a) 2.0C0
b) 1.6C0
c) 0.8C0
d) 2.5C0