C4 and cam plants are
a. adaptations to arid environments
b. adaptations to moist environments
c. more common than c3 plants
d. less efficient at water conservation than c3 plants

a. adaptations to arid environments

An example is a cactus which have specialised leaves and stems. 
Mostly are vascular plants.
Vascular tissue is responsible for transporting water and nutrients in plants. The correct answer in regards to the given question is option "c". The Vascular tissue consists of the Xylem and the Phloem. The main function of the Xylem is to transport water and minerals throughout all parts of the plant. Phloem on the other hand is responsible for transporting organic molecules that are larger in size. The vascular system, consisting on the Xylem and the Phloem runs from the roots of the plats through the branches and upto the leaves. It controls the total transportation of the water and nutrients.

It would be a rusting car, for sure