A 2.0μF air-filled parallel-plate capacitor is connected to a 12V battery. A slab of dielectric material with dielectric constant κ=4 is then slowly inserted so that it completely fills the space between the plates while the battery remains connected. What is the work done by an external agent in carrying out the insertion?
a) +0.864mJ
b) +0.432mJ
c) −0.432mJ
d) 0mJ