Photosynthesis takes in the carbon dioxide produced by all breathing organisms and reintroduces oxygen into the atmosphere. Photosynthesis is the process used by plants, algae and certain bacteria to harness energy from sunlight and turn it into chemical energy.
Your question is unclear but, based on the context, it is clear to me that the point that comes after C (point D) refers to the process of evaporation. Please try to post a picture of the diagram and make your question clearer next time.
