e taken soon after the illness sets in. As for colds, the sooner a person starts taking over-the-counter remedy, the sooner relief will come. 
　　The common cold and the flu are both caused by viruses. More than 200 viruses can cause cold symptoms, while the flu is caused by three viruses - flu A, B and C. There is no cure for either illness, but the flu can be prevented by the flu vaccine ( 疫苗), which is, for most people, the best way to fight the flu, according to the ALA. 
　　But if the flu does strike, quick action can help. Although the flu and common cold have many similarities, there are some obvious signs to look for. 
　　Cold symptoms such as stuffy nose, runny nose and scratchy throat typically develop gradually, and adults and teens often do not get a fever. On the other hand, fever is one of the characteristic features of the flu for all ages. And in general, flu symptoms including fever and chills, sore throat and body aches come on suddenly and are more severe than cold symptoms. 
　　The ALA notes that it may&

nbsp;be particularly difficult to tell when infants and preschool age children have the flu. It advises parents to call the doctor if their small children have flu-like symptoms. 
　　Both cold and flu symptoms can be eased with over-the-counter medications as well. However, children and teens with a cold or flu should not take aspirin for pain relief because of the risk of Reye syndrome(综合症) ，a rare but serious condition of the liver and central nervous system. 
   There is, of course, no vaccine for the common cold. But frequent hand washing and avoiding close contact with people who have colds can reduce the likelihood of catching one. 

21. According to the author, knowing the cause of the misery will help ____. 
　A) prevent people from catching colds and the flu 
　B) the patient obtain cheaper prescription drugs 
　C) the patient buy medicine over the counter 
　D) shorten the duration of the illness 
22. We learn from the passage that ____. 
　A) over-the-counter drugs can be taken to ease the misery caused by a cold or the flu 
　B)&

nbsp;one doesn’t need to take any medicine if he has a cold or the flu 
　C) aspirin should not be included in over-the-counter medicines for the flu 
　D) delayed treatment of the flu will harm the liver and central nervous system 
23. According to the passage, to combat the flu effectively, ____. 
　A) one should take medicine upon catching the disease 
　B) one should identify the virus which causes it 
　C) one should consult a doctor as soon as possible 
　D) one should remain alert when the disease is spreading 
24. Which of the following symptoms will distinguish the flu from a cold? 
　A) A sore throat. 
　B) A stuffy nose. 
　C) A dry cough. 
　D) A high temperature. 
25. If children have flu-like symptoms, their parents ____. 
　A) are encouraged to take them to hospital for vaccination 
　B) are advised not to give them aspirin 
　C) should watch out for signs of Reye syndrome 
  D) should prevent them from mixing with people running a fever 

Passage Four  
Questions 21 to 25 are based on the&nbs

p;following passage. 
　　Lead deposits, which accumulated in soil and snow during the 1960’s and 70’s, were primarily the result of leaded gasoline emissions originating in the United States. In the twenty years that the Clean Air Act has mandated unleaded gas use in the United States, the lead accumulation world-wide has decreased significantly. 
　　A study published recently in the journal Nature shows that air-borne leaded gas emissions from the United States were the leading contributor to the high concentration of lead in the snow in Greenland. The new study is a result of the continued research led by Dr. Charles Boutron, an expert on the impact of heavy metals on the environment at the National Center for Scientific Research in France. A study by Dr. Boutron published in 1991 showed that lead levels in arctic(北极的) snow were declining. 
　　In his new study, Dr. Boutron found the ratios of the different forms of lead in the leaded gasoline used in the United States were different from the ratios of European, Asian and Canadian gasolines and thus enabled scientists to different