Ans. Connecting 2nd bulb in series with the 1st one, it decreases the brightness of two bulbs. Adding additional bulbs in series with these two bulbs will further reduce the glow and brightness of each bulb. In fact, adding more bulbs in series circuit increases the resistance of the circuit. Hence, according to ohm's law (V = IR), current through each bulb will be reduced which, in turn decreases the brightness of each bulb.
Connecting 2nd bulb in parallel with 1st bulb, the battery of 2.5V will not affect the brightness of the 1st bulb. Rather, both bulbs will glow with same brightness as in parallel circuit, both bulbs have the same potential difference. Adding more bulbs will also not affect the brightness of each bulb. However, brightness of each bulb in parallel circuit is larger as compared to the brightness of bulbs in series circuit.