题目内容
Rubidium, potassium and carbon are three common elements used to date the history of Earth. The rates of radioactive decay of these elements are absolutely regular when averaged out over a period of time; nothing is known to change them. To be useful as clocks, the elements have to be fairly common in natural minerals, unstable but decay slowly over millions of years to form recognizable "daughter" products which are preserved minerals. For example, an atom of radioactive rubidium decays to form an atom of strontium (another element) by converting a neutron in its nucleus to a proton and releasing an electron, generating energy in the process. The radiogenic daughter products of the decay-in this case strontium atoms--diffuse away and are lost above a certain very high temperature. So by measuring the exact proportions of rubidium and strontium atoms that are present in a mineral, researchers can work out how long it has been since the mineral cooled below that critical "blocking" temperature. The main problems with this dating method are the difficulty in finding minerals containing rubidium, the accuracy with which the proportions of rubidium and strontium are measured, and the fact that the method gives only the date when the mineral last cooled below the blocking temperature. Because the blocking temperature is very high, the method is used, mainly for recrystallized (igneous or metamorphic) rocks, not for sediments--rubidium-bearing minerals in sediments simply record the age of cooling of the rocks which were eroded to form the sediments, not the age of deposition of the sediments themselves. Potassium decays to form (a gas) which is sometimes lost from its host mineral by escaping through pores. Although potassium-argon dating is therefore rather unreliable, it can sometimes be useful in dating sedimentary rocks because potassium is common in some minerals which form in sediments at low temperatures. Assuming no argon has escaped, the potassium-argon date records the age of the sediments themselves. Carbon dating is mainly used in archaeology. Most carbon atoms (carbon-12) are stable and do not change over time. However, cosmic radiation bombarding the upper atmospheres constantly interacting with nitrogen in the atmosphere to create an unstable form of carbon, carbon-14. What is the limitation of the rubidium method
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Consumers and producers obviously make decisions that mold the economy, but there is a third major (61) to consider the role of government. Government has a powerful (62) on the economy in at least four ways: Direct Services. The postal system, for example, is a federal system (63) the entire nation, as is the large and complex establishment. Conversely the construction and (64) of most highways, the (65) of the individual states, and the public educational systems, despite a large funding role by the federal government, are primarily (66) for by country or city governments. Police and fire protection and sanitation (67) are also the responsibilities of local government. Regulation and Control. The government regulates and controls private (68) in, many ways, for the (69) of assuring that business serves the best (70) of the people as a whole. Regulation is necessary in areas where private enterprise is granted a(n) (71) , such as in telephone or electric service. Public policy permits such companies to make a reasonable (72) , but limits their ability to raise prices (73) , since the public depends on their services. Often control is (74) to protect the public, as for example, when the Food and Drug Administration bans harmful drugs, or requires standards of (75) in food. In other industries, government sets guidelines to ensure fair competition without using direct control. Stabilization and Growth. Branches of government, including Congress and such entities as the Federal Reserve Board, attempt to control the extremes of boom and bust of inflation and depression, by (76) tax rates, the money supply, and the use of credit. They can also (77) the economy through changes in the amount of public spending by the government itself. Direct Assistance. The government provides many kinds of help to (78) and individuals. For example, tariffs (79) certain products to remain relatively free of foreign competition; imports are sometimes taxed so that American products are able to (80) better with certain foreign goods. In quite a different area, government supports individuals who cannot adequately care for themselves, by making grants to working parents with dependent children, by providing medical care for the aged and the indigent, and through social welfare system.
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