Answer:
The expanding gas is exerting pressure and doing work on the walls of the balloon causing it to inflate.
Step-by-step explanation:
"Dry ice" turns into gas in a process called "sublimation."
This is what happens when Bob placed several pieces of dry ice inside the empty water bottle. The room temperature allowed the dry ice to undergo sublimation. It then transformed into gas which exerted pressure on the walls of the balloon as it is expanding. This is the reason why the balloon became inflated.
So, this explains the answer.