Shown in the picture is a food web, which shows who eats the producers, and in turn who the predators are. It does not only show one chain, rather, a variety.
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I believe the correct answer from the choices listed above is the third option. It is oxygen that is a necessary reactant for cellular respiration to occur. Cellular respiration, in a sense, is breaking down sugar in the presence of oxygen. Hope this answers the question.
In agroecosystems, farmers practice monoculture. It is a practice of growing one type of crop in the same field year after year. Monoculture is an attempt to check herbivores from eating crops. As a result population of herbivores get reduced from the food webs. Using different types of agro-chemicals such as insecticides, pesticides, weedicides also reduces number species. Therefore, the most unusual characteristic of food webs in agroecosystems is lack of biodiversity.