What happens if you double the number of batteries used in a circuit? (assume all of the batteries are the same.)
a. the current doubles only.
b. the voltage doubles only.
c. the current decreases.
d. the voltage decreases.
e. the voltage and current doubles.

I think it is E. The voltage and current doubles. 

Answer;

E.The voltage and current doubles.

Explanation;From the Ohm's law, current through a conductor is directly proportional to the potential difference at constant temperature and pressure.Mathematically; I =V/R, where R is the resistance, I is the current and V is the voltage or the potential difference.Current is directly proportional to the voltage, thus and increase in voltage  causes an increases in the amount of current in a circuit and vice versa.Thus, if you double the number of batteries used in a circuit the both the current and voltage doubles.