题目内容：
根据材料，回答问题。Keeping Cut Flowers 1. While everybody enjoys fresh cut flowers around his house, few people know how to keep them for as long as possible. This may be done by keeping in mind a few simple facts.
2. An important thing to remember about cut flowers is that they are sensitive to temperature. For example, studies have shown that cut carnations (康乃馨) retain their freshness eight, times longer when kept at 12~C than when kept at 26~C. Keeping freshly harvested flowers at the right temperatures is probably the most important aspect of flower care.
3. Flowers are not intended by nature to live very long. Their biological purpose is simply to attract birds or insects, Such as bees, for pollination (授粉)fg). After that, they quickly dry up and die. The process by which flowers consume oxygen and produce carbon dioxide (二氧化碳), called respiration (呼吸), generates the energy the flower needs to give the flower its shape and colour. The making of seeds also depends on this energy. While all living things respire, flowers have a high level of respiration. A result of all this respiration is heat, and for flowers the level of heat relative to the mass of the flower is very high. Respiration also brings about the eventual death of the flower. Thus the greater the level of respiration, the sooner the flower dies.
4. How, then, to control the rate at which flowers die? By controlling respiration. How is respiration controlled? By controlling temperature we know that respiration produces heat, but the reverse is also true. Thus by maintaining low temperatures, respiration is reduced and the cut flower will age more slowly.
5. Another vital factor in keeping cut flowers is the quality of the water in which they are placed. Flowers find it difficult to "drink" water that is dirty or otherwise polluted. Even when water looks and smells clean, it almost certainly contains harmful substances that canendanger the flowers. To rid the water of these unwanted substances, household chlorine bleach (含氯漂白剂) can be used in small quantities. It is recommended that 15 drops of chlorine bleach ( at 4% solution) be added to each litre of water. The water and solution should also be replaced each day.
Paragraph 2___ A．Control of Respiration
B．Beauty of Fresh Cut Flowers
C．Role of Respiration
D．Most Important Aspect of Flower Care
E．Need for Clean Water
F．Ways of Stopping Respiration
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