第2部分:阅读判断(第16~22题,每题1分,共7分) $ f$ U% @& Y3 ?: H 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。 / h) j* {6 e8 k% NRadiocarbon Dating + g. P: f9 @: ]! Z2 j" Y Nowadays scientists can answer many questions about the past through a technique 1 V# x" t }/ pcalled radiocarbon(放射性碳).or carbon-14,dating One key to understanding how and 3 O1 R+ _$ M8 X N" ^why something happened is to discover when it happened 8 A! |) o2 C' G- R; @1 oRadiocarbon dating was developedinthelatel940s by physicist Willard F Libby at/ h0 {6 [' L; ~; y7 l1 I
the University of Chicago An atom of ordinary carbon called carbon-12 has six protons 7 M$ ]+ l! h: `( s ^ (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 : f" j9 e5 `) p3 N, N+ k) r4 a+ k 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 dated3 m* i J" l. C6 E8 g' m
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 years4 {. u) F* f' {0 T* c- w; P4 A
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.