What Is the Primary Reason for Plants Closing Their Stomata

What is the primary reason for plants closing their stomata when it gets dark?

A) This prevents the acquisition of excessive moisture.
B) The plant is able to seal in CO₂ when its stomata are closed.
C) This prevents loss of oxygen while the plant is "resting."
D) This protects the plant against predation by microscopic pests.
E) CO₂ is not needed when photosynthesis is not taking place.