The Thick, Green Stems and Spike- Like Leaves of Cacti

The thick, green stems and spike- like leaves of cacti and other drought- resistant bushes are adaptations to

A) increase gas exchange in oxygen- poor environments.
B) collect light in environments that have limited amounts of light.
C) take up atmospheric nitrogen in nutrient- poor environments.
D) conserve water in dry environments.
E) speed up heating and cooling in environments that have little temperature fluctuation.