How does the movement of muscles attached to the ribcage and diaphragm initiate expiration?

The muscles relax, which which flattens the diaphragm and increases the volume of the chest cavity.
The muscles relax, which pulls the ribcage downward and decreases the volume of the chest cavity.
The lungs contract and push the air from the lungs.
The lungs relax and pull the air into the lungs.