单选题 Radiocarbon Dating
Nowadays scientists can answer many questions about the past through a technique called radiocarbon (放射), or carbon-14, dating. One key to understanding how and why something happened is to discover when it happened.
Radiocarbon dating was developed in the late 1940s 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 a 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(氮)-14,or N-14,interacts with cosmie 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 atraosphere. 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 ,resent 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.

【正确答案】 A

【答案解析】[解析] 关键词为organism(有机体)和decay(衰退)。依据此关键词，可在文章最后一段找到相关叙述：once an organism dies it stops taking in carbOn in any form，and the C-14 already present begins to decay(一旦有机体消亡，便会停止吸收任何形式的碳，现有的碳14也会随之开始衰退)，与句意相符，故此题为“正确”的。