Which of the following is not correct? cam and c4 compared to c3 plants.
a. are more economical in their use of water
b. open there stomata more frequently and for longer lengths of time
c. are typically found in hotter and more arid areas
d. reach their maximum photosynthetic rates at high radiation intensities

b. open their stomata more frequently and for longer lengths of time

Explanation:

One of the functions of the stomata is to take in carbon dioxide for photosynthesis. C4 and CAM plants do not open their stomata more frequently than the C3 plants. The C4 and CAM plant concentrate carbon dioxide around RuBisCo enzyme in their bundle sheath cells. The efficient incorporation of CO2 allows them to keep the stomata closed for most of the time.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.