When C and D systems are in position I, the total electric potential energy is zero, and when they move to infinity, the total electric potential energy is also zero. Because the sum of the electric potential energy of the two balls decreases qcld in the process of moving from position I to position II and increases qcld in the process of moving from position II to infinity, the system overcomes the electric field force to do work qQLdC. So, C is wrong and D is right.
So choose AD.