You reposition the two plates of a parallel-plate capacitor so that the distance between them doubles. There is vacuum between the plates. If the charges +Q and -Q on the two plates are kept constant in this process, which of the following statements is incorrect?
A) The potential difference between the two plates doubles.
B) The electric field between the two plates keeps the same.
C) The potential energy stored in this capacitor doubles.
D) The capacitance of this capacitor doubles.