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  3. Chlorine is a mainstay in most swimming pools in the United States, and is used to eradicate bacteria. A bacteria-free pool will

Chlorine is a mainstay in most swimming pools in the United States, and is used to eradicate bacteria. A bacteria-free pool will

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问题     Chlorine is a mainstay in most swimming pools in the United States, and is used to eradicate bacteria. A bacteria-free pool will usually ensure that swimmers won’t contract a serious illness if they spend time in, and possibly ingest, the water. When chlorine is added to pool water, a chemical reaction occurs whereby the chlorine breaks down into a legion of chemicals that combine to kill bacteria that may be in water. These chemicals, specifically hypochlorous acid and hypochlorite ion, eliminate bacteria by annihilating and then oxidizing the abhorrent cells. When this chemical reaction occurs, the baeteria are rendered essentially harmless. This chemical reaction doesn’t occur immediately — individual components within chlorine take varying amounts of time to interact with the bacteria, creating an almost time-release-type of kill in the pool water.
    Chlorine must be regularly replenished. As they react, the hypochlorous acid and hypochlorite ion combine to create a particular acceptable pH level in the pool water. A pH is a measure of the alkalinity and acidity present; a pH value of 7 represents a neutral value, and 7. 4 is the pH value of the human tear. The pH ratio must be regularly monitored to ensure a consistent optimum ram — one usually kept within the spectrum of 7 to 8 as measured by a reliable pH measuring kit. As the chemicals in chlorine complete their reactions, they break down and are no longer effective in keeping the pool water neutralized. Additional chlorine must be added to water at specific times. Outdoor pools that are in direct sunlight need to have water added more often because the chemical reaction time for chlorine is increased and its effectiveness is reduced in these conditions. Since the hypochlorite ion is an especially potent chemical capable of fading fabrics and paint, most people are meticulous about rinsing chlorine from potentially affected areas. Although chlorine is inexpensive and widely available, some people find the bleach smell to be repugnant and the distinct aroma present at many swimming pools to be overwhelming. These people may not be able to enjoy aquatic pursuits in those pools that employ chlorine as a cleansing agent. Other people may find that inhaling chlorine at the high levels present around some pools may be difficult to tolerate. Still others may experience skin irritation after swimming in a pool containing chlorine. Although there are other chemicals and compounds available to clean swimming pools, they may be prohibitively expensive or incapable of killing all types of bacteria present in the water.
According to the passage, a stable pH value in pool water is dependent upon which of the following?
选项 A、Monitoring and adding chlorine when warranted.
B、Ensuring that there is only a minimum amount of bacteria in the water.
C、Paying close attention to the reactions of chemicals in the pool water.
D、Ensuring that water is protected from direct sunlight.
答案A
解析 细节题。从第二段第一句话“Chlorine must be regularly replenished“以及”TheoH ratio must be regularly monitored to ensure a consistent optimum…”可知正确答案是A。
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