The answer is B) The intrapulmonary volume must increase.
It is known that gas travels from the area with high pressure to the area of low pressure. When we inspire, our diaphragm and intercostal muscles contract, so the volume of the chest increases. This volume increase leads to a decrease of alveoli' air pressure below atmospheric pressure. Now, when the pressure inside the alveoli is decreased comparing to atmospheric pressure, gas from the air enters lungs and reaches alveoli where the pressure is the lowest.
