a) During sextion B-C we can see that theintrapulmonary pressure is lower than the standard atmospheric pressure. This tells us that the air is moving into the lungs, therefore they are being filled. For this to happen, the diaphragm contracts itself and gets lower, while the rib cage opens and expands, providing more space for the lungs to get bigger as they are being filled with air.
During section C-D the intrapulmonary pressure is higher than the standard atmospheric pressure. This tells us that the air is leaving the lungs, and they are getting empty. When this happens, the diaphragm relaxes and gets back to its place, while the rib cage retracts and closes in, forcing the lungs to shrink and to expell the air that was inside.
b) The pulmonar volume refers to the amount of air that's inside the lungs in a specific moment. In section B-C the volume will get higher and higher as the lungs are filled with air, and in section C-D the volume gets lower as the air leaves the lungs.
c) During inspiration, the volume of the thoracic cavity increases (as there is air flowing inside the lungs) and the intrapulmonary pressure decreases (as the volume increases being filled with air, and when there is a lower pressure environment than the atmospheric pressure the air will be attracted to the place where the pressure is lower). During expiration, the volume decreases (as the air is leaving the lungs) and the pressure increases (as the volume decreases because the air is leaving the lungs, and when there is a higher pressure environment than the atmospheric pressure the air will be attracted to the place where the pressure is lower - in this case, out of the lungs).