21. For this LSAT main point question, the author seeks to answer the question posed in lines 15 to 16, and the main point of the passage is that the C-4 process causes maize's high productivity. Choice (A) is correct.
(B) refers "nutrients" but the passage refers to the crop. Also, it's not all C-4 plants that have shaped culture, but, at least as far as the passage states, just maize.
(C) states that the C-4 makes more efficient use of atmospheric hydrogen, yet the passage states that C-4 stops oxygen from impairing photosynthesis - see lines 36 to 37.
For (D), all plants contain rubisco - see the second paragraph. And, rubisco is part of the C-4 process but not in the ability of C-4 plants to stop the negative effects - see lines 45 to 47.
(E) states "mechanisms", which is plural, but the passage only refers to one, C-4.
22. This passage organization question asks about the final two paragraphs (and excludes the first paragraph). Two choices -- (A) and (D) refer to the first paragraph and may be eliminated because of this.
(B) is correct because the final two paragraphs have a biochemical process (photosynthesis), a hindrance (oxygen binds to the enzyme), solution (C-4), and the crop (maize).
(C) is incorrect because of "processes" (only one - photosynthesis) and "two ways" (only one - C-4).
(D) is incorrect because it begins with "cultural phenomenon", which only appears in the first paragraph.
(E) is wrong because the author does not provide an argument within the passage -- s/he provides factual knowledge.
23. Approach this as an inference question. (B) is the correct choice because an enzyme with such a property eliminates the need for C-4 photosynthesis.
Choice (A) is incorrect because plants want to release oxygen (lines 23 to 24), not keep it.
(C) is wrong because, within C-4 at least, rubisco is not inside the vascular structures but rather within the bundle sheath cells, and, the passage does not tell us where rubisco is in non-C-4 plants.
(D) is wrong because in C-4, the bundle sheath cells surround the vascular structure, but if the specialized structures do, then we have no clue what would happen.
(E) is incorrect because this may slow photosynthesis -- see lines 24 to 28. For (E) to be correct, it would need "react" replaced with "bind", and "carbon dioxide" replaced with "oxygen".
24. This LSAT purpose of phrase question asks about "all other gases", which includes carbon dioxide, which therefore must change to a nongas molecule -- see lines 50 to 54. (D) accurately describes this.
(A) is wrong because the cause of atmospheric conditions that cause excess oxygen is never discussed.
(B) is wrong because there is no claim that gases besides oxygen interfere with photosynthesis.
(C) is wrong because there is no claim that non-C-4 plants use more gases.
(E) is wrong because there is no claim that oxygen levels remain constant in C-4 plants.