A battery powers two identical lamps, L1 and L2, connected in series. A switch is wired so that closing it bypasses (short-circuits) L2. What happens to the brightness of L1 when the switch is closed?
a) L1 goes out because the potential difference across it becomes zero.
b) L1's brightness is unchanged because its resistance is unchanged.
c) L1 becomes brighter because the circuit current increases.
d) L1 becomes dimmer because the circuit current decreases.