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第2部分：阅读判断（第16～22题，每题1分，共7分）
下面的短文后列出了7个句子，请根据短文的内容对每个句子做出判断：如果该句提供的是正确信息，请选择A；如果该句提供的是错误信息，请选择B；如果该句的信息文中没有提及，请选择C.
Radiocarbon Dating
Nowadays scientists can answer many questions about the past through a technique called radiocarbon （放射性碳）， or carbon-14， dating. One key to understanding how and by something happened is to discover when it happened.
Radiocarbon dating was developed in the late 1940s by physicist Willard F. Libby at he University of Chicago. An atom of ordinary carbon， called carbon-12， has six protons（中子） and six neutrons （质子） in its nucleus. Carbon-14， or C-14， is a radioactive， unstable form of carbon that has two extra neutrons （原子核）。 It returns to a more stable form of carbon through a process called decay （衰减）。 This process involves the loss of he extra neutrons and energy from the nucleus.
In Libbys radiocarbon dating technique， the weak radioactive emissions （放射） from his decay process are counted by instruments such as a radiation detector and counter. he decay rate is used to determine the proportion of C-14 atoms in the sample being dated.
Carbon-14 is produced in the Earths atmosphere when nitrogen （氮）-14， or N-14，
interacts with cosmic rays （宇宙射线）。 Scientists believe since the Earth was formed， the mount of nitrogen in the atmosphere has remained constant. Consequently， C-14 formation is thought to occur at a constant rate. Now the ratio of C-14 to other carbon toms in the atmosphere is known. Most scientists agree that this ratio is useful for dating items back to at least 50，000 years.
All life on Earth is made of organic molecules （分子） that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio of C-14
atoms to other carbon atoms in their tissues （组织）。 Once an organism （有机体） dies it tops taking in carbon in any form， and the C-14 already present begins to decay. Over time the amount of C-14 in the material decreases， and the ratio of C-14 to other carbon toms goes down. In terms of radiocarbon dating， the fewer C-14 atoms in a sample， the older that sample is.
16 Nowadays many scientists depend on radiocarbon for dating age-old objects
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17 The radiocarbon dating technique is only about 40 years old
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18 An atom of ordinary carbon has six protons and eight neutrons
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19 Radar is used to determine the characteristics of radiocarbon
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20 Radiocarbon is reliable in dating an object back to at least 50，000 years.
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21 The C-14 in an organism begins to decay when it dies
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22 The half-life of C-14 is about 25，000 years.
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