Which statement best describes the motion of charge carriers in a metal wire when the net electric current in the wire is zero?
a) Electrons oscillate back and forth about fixed positions so that, over time, no net charge crosses a cross-section.
b) All charge carriers are at rest within the conductor.
c) Charge carriers move randomly in all directions, and the random motions cancel so that no net charge crosses a cross-section over time.
d) Positive ions drift one way while electrons drift the other with equal magnitudes, resulting in zero net current.