In succulent plants,the stomata open at night and close during the day. Which among the following would be the best hypothesis to explain the mechanism of stomatal action at night only?

• A
  $CO_2$ accumulates,reduces $pH$,stimulates enzymes resulting in the accumulation of sugars.
• B
  Increase in $CO_2$ concentration,conversion of starch into organic acids resulting in increased concentration of cell sap and $K^+$ transport.
• C
  Low $CO_2$ concentration accumulates organic acids resulting in the increased concentration of cell sap.
• D
  $CO_2$ is used up,increase in $pH$ results in the accumulation of sugars.