Passage 4
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 bacteria 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 is 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.
16.According to the passage，a stable pH value in pool water is dependent upon.
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
17.The author’s primary purpose is to____.
A.offer an unbiased opinion of chlorine’s efficacy
B.provide an argument for using chlorine
C.discuss the chemical reaction taking place within a pool
D.show how chlorine can keep pool water safe
18.Based on the passage，which statement is probably NOT true of chlorine？(https://www.daowen.com)
A.It can be harmful if swallowed.
B.Occasional high levels in pool water are probably not harmful.
C.It can kill most common types of pool water bacteria.
D.A pool’s size will dictate the amount needed.
19.The author includes the information about the pH of human tears mainly to____.
A.give the readers a base of reference
B.personalize the passage to enhance interest
C.provide a visual image for the readers
D.answer the readers’anticipated question
20.All of the following statements are true about chlorine EXCEPT that____.
A.chlorine’s efficacy is affected by sunlight
B.chlorine is made up of two chemicals：hypochlorous acid and hypochlorite ion
C.chlorine is usually said to be malodorous
D.chlorine levels must be routinely assessed in pool water