Answer:
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.