The amount of energy for a coulomb of charge at one end of a conductor is 40 joules. The amount of energy for a coulomb of charge at the other end of the conductor is 20 joules. The potential difference across the ends of the conductor is ______. 
 A.) 60 joules per coulomb 
 B.) 60 joules 
 C.) 20 joules per coulomb 
 D.) 20 joules