Two identical air-filled parallel-plate capacitors C1 and C2, each with capacitance C, are connected in series to a battery that has voltage V. While the two capacitors remain connected to the battery, a dielectric with dielectric constant K>1 is inserted between the plates of one of the capacitors, completely filling the space between them. Let U0 be the total energy stored in the two capacitors without the dielectric and U be the total energy stored after the dielectric is inserted.

a) What is the ratio U/U0?
b) Does the total stored energy increase, decrease, or stay the same after the dielectric is inserted?