Which of the following statements are true? (More than one answer may be correct - select all that are true). Select one or more:
a. In a circuit, current is delivered by the positive terminal of a battery, and it is used up by the time it returns to the negative terminal of the battery.
b. In order to maintain a steady flow of current in a conductor, a steady force must be maintained on the mobile charges.
c. When an electric field is applied to a conductor, the free electrons move only in the direction opposite the applied electric field.
d. By convention, the direction of a current is taken to be the direction of flow for negative charges.
e. Current is the total amount of charge that passes through a conductor's full cross section at any point per unit of time.