A parallel-plate capacitor is charged to a potential difference of 200V and then disconnected from the battery. The initial electric field magnitude between the plates is E0. A dielectric with dielectric constant κ=2.5 is inserted so that it completely fills the space between the plates. What is the ratio of the new electric field magnitude E to the original field magnitude E0?
a) 1.0
b) 0.16
c) 0.40
d) 2.5