Answer
a. They get dimmer. They stay the same in brightness.
b. When the bulbs are in series, the battery lasts longer.
Work Step by Step
a. When bulbs are added in series, the circuit resistance increases, which decreases the current that the battery supplies. The voltage across each of them drops as another bulb is added, so each bulb becomes dimmer.
When bulbs are added in parallel, each new bulb still has the same voltage drop as the bulbs that were already there. The bulbs do not change brightness as more bulbs are added.
b. The battery lasts longer in the series case, because all the bulbs become dimmer and consume less power as new bulbs are added.
