Passage ThreeQuestions 21 to 25 are based on the following passage.Crying is hardly an activity encouraged by society. Tears, be they of sorrow, anger, on joy, typically make Americans feel uncomfortable and embarrassed. The shedder of tears is likely to apologize, even when a devastating (毁灭性的) tragedy was the provocation. The observer of tears is likely to do everything possible to put an end to the emotional outpouring. But judging form recent studies of crying behavior, links between illness and crying and the chemical composition of tears, both those responses to tears are often inappropriate and may even be counterproductive.Humans are the only animals definitely known to shed emotional tears. Since evolution has given rise to few, if any, purposeless physiological response, it is logical to assume that crying has one or more functions that enhance survival.Although some observers have suggested that crying is a way to elicit assistance form others (as a crying baby might from its mother), the shedding of tears is hardly necessary to get help. Vocal cries would have been quite enough, more likely than tears to gain attention, So, it appears, there must be something special about tears themselves.Indeed, the new studies suggest that emotional tears may play a direct role in alleviating stress, University of Minnesota researchers who are studying the chemical composition of tears have recently isolated two important chemicals from emotional tears. Both chemicals are found only in tears that are shed in response to emotion. Tears shed because of exposure to a cut onion would contain no such substance.Researchers at several other institutions are investigating the usefulness of tears as a means of diagnosing human ills and monitoring drugs.At Tulane University’s Teat Analysis Laboratory Dr. Peter Kastle and his colleagues report that they can use tears to detect drug abuse and exposure to medication(药物), to determine whether a contact lens fits properly of why it may be uncomfortable, to study the causes of “dry eye” syndrome and the effects of eye surgery, and perhaps even to measure exposure to environmental pollutants.At Columbia University Dt. Liasy Faris and colleagues are studying tears for clues to the diagnosis of diseases away from the eyes. Tears can be obtained painlessly without invading the body and only tiny amounts are needed to perform highly refined analyses.It is known from the first paragraph that ________.