The stomata on a saguaro cactus must open to let in carbon dioxide from the atmosphere. When each stoma opens, however, water can escape from the plant. How
does the saguaro minimize the loss of water when stomata open?

A. Guard cells open and close the stomata rapidly during the day to let as little
water as possible escape.
B. B. Guard cells open the stomata only at night, when the air temperature is cooler
and the humidity somewhat higher.
C. Guard cells always keep the stomata open slightly, so that carbon dioxide can
flow in but very little water can flow out.
D. The saguaro has fewer stomata than any other plant, an adaptation that prevents
water loss through transpiration.