The answer: intrapulmonary pressure; atmospheric pressure
Intrapulmonary pressure refers to the pressure that exist in your pleural cavity. Atmospheric pressure, refers to the pressure that exist in earth's atmosphere. Air will move from high pressure to low pressure area. So, in order for the air to sucked in, we will expand our lung to reduce our intrapulmonary pressure and allow air to move from our environment into our body.
I believe the answer is a, Napoleon's invasion of Spain, but correct me if I'm wrong. Hope this helps!