阅读判断（第1～7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 why something happened is to discover when it happened Radiocarbon dating was developedinthelatel940s by physicist Willard F Libby at the University of Chicago An atom of ordinary carbon called carbon-12 has six protons (质子)and six neutrons(中子)in its nucleus(原子核)Carbon-14，or C-14，is a radioactive unstable form of carbon that has two extra neutrons .It returns to e more stable form of carbon through a process called decay(衰减)This process involves the loss of the extra neutrons and energy from the nucleus. In Libby’s radiocarbon dating technique，the weak radioactive emissions(放射)from this decay process are counted by instruments such as a radiation detector and counter．The decay rate is used to determine the proportion of C-14 atoms in the sample being dated .Carbon-14 is produced in the Earth’s atmosphere when nitrogen(氨)-14．or N-14，interacts with cosmic rays(宇宙射线)Scientists believe since the Earth was formed，the amount 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 atoms 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 stops 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 atoms goes down In terms of radiocarbon dating．the fewer C-14 atoms in a sample，the older that sample is.
1 Nowadays many scientists depend on radiocarbon for dating age-old objects
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2 The radiocarbon dating technique is only about 40 years old
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3 An atom of ordinary carbon has six protons and eight neutrons
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4 Radar is used to determine the characteristics of radiocarbon
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5 Radiocarbon is reliable in dating an object back to at least 50,000 years
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6 When an organism dies．the C-14 in it begins to decay．
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C Not mentioned
7 The half-life of C-14 is about 25,000 years
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C Not mentioned
