Many cactus plants, such as the species shown in the illustration, use a special type of photosynthesis called cam photosynthesis. this specialized process allows the cactus plant to keep the stoma or leaf openings closed during the day. most other plant species keep their stoma open during daylight hours. which is the most likely reason that cactus use this special photosynthetic process? a) retain oxygen b) conserve water c) avoid insect predators d) conserve energy in the form of atp

B because cactus' are usually in a dry environment, they close their stomata during the day to keep water vapor from leaving the plant. Instead they do their processes at night to avoid extreme water loss.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.