Points c and d would have spring tides.
Photosynthesis is the process by which plants use light energy to convert carbon dioxide and water into sugars...Respiration occurs when glucose combines with oxygen to produce useable cellular energy. This energy is used to fuel growth and all of the normal functions.
hope this helps
I do think that they close to not allow too much of either water or air from entering.
the hollow tube allows water to flow easily.
https://www.bbc.co.uk/bitesize/guides/z28g7p3/revision/1