Vowels can be classified into pure vowels and diphthongs.

听力原文： We know that we have to pay for what we get. If we buy food, we know there will be a bill to pay. These are private bills. But there are also public bills that must be paid. Public bills are paid by the government. In turn, we get these services through taxes. What would happen if everyone in the city stopped paying taxes? The water supply would stop; water might become unclean and impure. The streets might not be cleaned. There would be no police force to protect people and property. Schools would be closed. People would become sick and diseases might spread. We should not want to live in such a city. We all want pure water and food, clean streets and good schools. We want the police to prevent crime. The chief duty for every government is to protect persons and property. More than three quarters of the money spent by our government is used for this purpose. The next largest amount of public money goes to teach and train our citizens. Billions of dollars each year are spent on schools and libraries. Public money is used to pay the teachers and all the public officials. A large number of public funds are also spent on roads.

The greatest part of necessary funds is raised by taxes. A tax is money that we all must pay to support the government. The law orders us to pay taxes. We have no choice in the matter. Almost everyone pays some taxes in one form. or another.

(30)

A．By taxing public services.

B．By providing needed services.

C．By making people pay for private bills.

D．By collecting taxes.

Community cancer dusters are viewed quite differently by citizen activists and by epidemiologists. Environmentalists and concerned local residents, for instance, might immediately suspect environmental radiation as the culprit when a high incidence of cancer cases occurs near a nuclear facility. Epidemiologists, in contrast, would be more likely to say that the incidences were "inconclusive" or the result of pure chance. And when a breast cancer survivor, Lorraine Pace, mapped 20 breast cancer cases occurring in her West Islip, Long Island, community, her rudimentary research efforts were guided more by hope—that a specific environmental agent could be correlated with the cancers—than by scientific method.

When epidemiologists study clusters of cancer cases and other noncontiguous conditions such as birth defects or miscarriage, they take several variables into account, such as background rate (the number of people affected in the general population) , duster size, and specificity (any notable characteristics of the individual affected in each case). If a cluster is both large and specific, it is easier for epidemiologists to assign blame. Not only must each variable be considered on its own, but it must also be combined with others. Lung cancer is very common in the general population. Yet when a huge number of cases turned up among World War II shipbuilders who had all worked with asbestos, the size of the cluster and the fact that the men had similar occupational asbestos exposures enabled epidemiologists to assign blame to the fibrous mineral.

Although several known carcinogens have been discovered through these kinds of occupational or medical clusters, only one community cancer duster has ever been traced to an environmental cause. Health officials often discount a community's suspicion of a common environmental cause because citizens tend to include cases that were diagnosed before the afflicted individuals moved into the neighborhood. Add to this is the problem of cancer's latency. Unlike an infectious disease such as cholera, which is caused by a recent exposure to food or water contaminated with the cholera bacterium, cancer may have its roots in an exposure that occurred 10 to 20 years earlier.

Do all these caveats mean that the hard work of Lorraine Pace and other community activists is for nothing? Not necessarily. Together with many other reports of breast cancer clusters on Long Island, the West Islip situation highlighted by Pace has helped epidemiologists lay the groundwork for a well-designed scientific study.

The "hope" mentioned in Paragraph 1 refers specifically to Pace's desire to______.

A．help reduce the incidence of breast cancer in future generations

B．improve her chances of surviving breast cancer

C．determine the cause responsible for her own breast cancer case

D．identify a particular cause for the breast cancer cases in West Islip