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About fifty years ago, plant physiologists set out to grow roots, by themselves in solutions in laboratory flasks. The scientists found that the nutrition of isolated roots was quite simple. They required sugar and usual minerals and vitamins. However, they did not require organic nitrogen compounds. These roots got along fine on mineral inorganic nitrogen. Roots are capable of making their own proteins and other organic compounds. These activities by roots require the energy of course. The process of respiration uses sugar to make the high-energy compound ATP, which drives the biochemical reactions. Respiration also requires oxygen. Highly active roots require a good deal of oxygen. 

1.What is the main topic of the passage ?

• A. The relationship between a plant's roots and its shoots
• B. The relationship between a plant's roots and its shoots
• C. The relationship between a plant's roots and its shoots
• D. The relationship between a plant's roots and its shoots

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