Which one of the following describes a possible reason for the opening and closing of the guard cells?

• A
  Stomata open in daylight because photosynthesis occurs in the guard cells producing sugar resulting in higher osmotic pressure.
• B
  Stomata open in daylight because guard cells have chloroplasts and the epidermal cells do not,and so a differential osmotic pressure arises.
• C
  Stomata open in daylight because starch is changed to osmotically active substances which increase the turgidity of the guard cells.
• D
  Stomata close in darkness because the starch produced in daylight by photosynthesis is changed into sugars and translocated from the guard cells.