Question ID:PT49 S3 Q24 Passage:Every culture that has adopted the cultivation of maize‚ also known as corn‚ has been radically changed by it. This crop reshaped the cultures of the Native Americans who first cultivated it, leading to such developments as the adoption of agrarian and in some cases urban lifestyles, and much of the explosion of European populations after the fifteenth century was driven by the introduction of maize together with another crop from the Americas, potatoes. The primary reason for this plant's profound influence is its sheer productivity. With maize, ancient agriculturalists could produce far more food per acre than with any other crop, and early Central Americans recognized and valued this characteristic of the plant. But why are maize and a few similar crops so much more bountiful than others? Modern biochemistry has revealed the physical mechanism underlying maize's impressive productivity.To obtain the hydrogen they use in the production of carbohydrates through photosynthesis, all plants split water into its constituent elements, hydrogen and oxygen. They use the resultant hydrogen to form one of the molecules they need for energy, but the oxygen is released into the atmosphere. During photosynthesis, carbon dioxide that the plant takes in from the atmosphere is used to build sugars within the plant. An enzyme, rubisco, assists in the sugar-forming chemical reaction. Because of its importance in photosynthesis, rubisco is arguably the most significant enzyme in the world. Unfortunately, though, when the concentration of oxygen relative to carbon dioxide in a leaf rises to a certain level, as can happen in the presence of many common atmospheric conditions, oxygen begins to bind competitively to the enzyme, thus interfering with the photosynthetic reaction.Some plants, however, have evolved a photosynthetic mechanism that prevents oxygen from impairing photosynthesis. These plants separate the places where they split water atoms into hydrogen and oxygen from the places where they build sugars from carbon dioxide. Water molecules are split, as in all plants, in specialized chlorophyll-containing structures in the green leaf cells, but the rubisco is sequestered within airtight tissues in the center of the leaf. The key to the process is that in these plants, oxygen and all other atmospheric gases are excluded from the cells containing rubisco. These cells, called the bundle sheath cells, surround the vascular structures of the leaf‚ structures that function analogously to human blood vessels. Carbon dioxide, which cannot enter these cells as a gas, first undergoes a series of reactions to form an intermediary, nongas molecule named C-4 for the four carbon atoms it contains. This molecule enters the bundle sheath cells and there undergoes reactions that release the carbon dioxide that will fuel the production of carbohydrates (e.g., sugars). Taking its name from the intermediary molecule, the entire process is called C-4 photosynthesis. Such C-4 plants as sugar cane, rice, and maize are among the world's most productive crops. Stem:The author's reference to "all other atmospheric gases" near the middle of the last paragraph plays which one of the following roles in the passage? Correct Answer Choice:DChoice A:It indicates why certain atmospheric conditions can cause excess oxygen to build up and thus hinder photosynthesis in non-C-4 plants as described in the previous paragraph. Choice B:It supports the claim advanced earlier in the paragraph that oxygen is not the only atmospheric gas whose presence in the leaf can interfere with photosynthesis. Choice C:It supports the conclusion that non-C-4 photosynthesis makes use of several atmospheric gases that C-4 photosynthesis does not use. Choice D:It explains why carbon dioxide molecules undergo the transformations described later in the paragraph before participating in photosynthesis in C-4 plants. Choice E:It advances a broader claim that oxygen levels remain constant in C-4 plants in spite of changes in atmospheric conditions.