Inertia is responsible for:
1. water being made out of atoms.
2. the reason we must wear seatbelts.
3. a bird flying.

Inertia is the reason that people in cars need to wear seat belts. If we didn't wear seat belts and came to an abrupt stop, you would go straight through the windshield. 

Hope this helps!

2. the reason we must wear seatbelts
Your answer is 2 because, since inertia is the resistance of any physical object to any change in its state of motion, if the car stopped, us (physical object) would keep going since our bodies would resist to stop even if the car did because of inertia.