forces

I am having difficulty understanding the explanation of the answers in the EK book. Would someone be able to explain how they they worked out the answers to the following questions please ? Thanks !

If the voltage across the plates were reversed what would happen to the oil drops between the plates from the Millikan oil droplet experiment? A- They would accelerate upward B- They would accelerate downward C- They would move with constant velocity upward D- They would move with constant...