How does air pressure control expiration?

Question

How does air pressure control expiration? 
Select one:
a. A decrease in air pressure within the lungs allows air to flow in.
b. An increase in air pressure within the lungs allows air to flow out.
c. A decrease in air pressure within the lungs causes them to contract.
d. An increase in air pressure within the lungs causes them to expand.

Answer 1

A gas will flow from an area of high pressure, to an area of low pressure.  The diaphragm expands and contracts to manipulate the pressure within the lungs to either be greater or less than the atmospheric pressure.