Which muscles undergo contraction during normal expiration?

Which one of the following is a possibility for most of us in regard to breathing,by making a conscious effort?

During expiration,the diaphragm becomes

What happens to the diaphragm muscles during the process of inspiration?

Explain the process of inspiration under normal conditions.

State the sequence of events for inspiration (breathing in).
$I$ - Increase in lung volume
$II$ - Decrease in intrapulmonary pressure
$III$ - Contraction of respiratory muscles
$IV$ - Increase in thoracic volume
$V$ - Air rushes into the lungs