A student connects a simple series circuit consisting of a d.c. cell, an ammeter, and a resistor, as shown in the diagram.
Which of the following statements correctly describes the fundamental physical characteristic of a direct current (d.c.) in this circuit?
The net flow of charge carriers is in one direction only.
The magnitude of the current must remain perfectly constant over time.
The charge carriers continuously oscillate back and forth about a fixed position.
The current periodically reverses its direction with a constant frequency.