Question

阅读下列短文，从所给的四个选项中，选出最佳答案

　The doctor pointed to some bones on his desk．“They' re a man' s foot bones,”he said．“They were found deep inside the cave, and I' ve been examining them．The C14 test shows that they are about 12, 000 years old．”

　“What' s the C14 test?”I asked．

　“It' s a way of finding the age of some things that had been done long ago．The‘C’means carbon (碳), and carbon is found in all living things．It is also found in the remains of things that lived and died thousands or millions of years ago．Plants and animals take carbon into their bodies from the air and their food．It stays in their remains．

　“Well, there are two kinds of carbon, called Garbon 12 (C12) and Carbon 14 (C14)．Both are taken in together; but in living things C12 in one trillion (1012) times more plentiful (丰富的、大量的) than C14．The two kinds are different in one important respect．

　“C14 is radio-active (放射性的): it gives out rays of energy at a fixed frequency．As it does so, it changes to C12．

　In fact, exactly a half of the C14 loses its energy and becomes C12 in 5, 600 years．Then in the next 5, 600 years, half of the remaining C14 becomes C12．And so on．We say that C14 has a half-life (半衰期) of 5, 600 years．”

“You must have found some C14 in these bones, then,”I said．

　“Yes, I did．But the C12 is now four million times more plentiful．About a quarter of the C14 is left．That means, near enough that two half-lives have passed．”The doctor picked up one of the brown toe bones．“It' s 12, 000 years since this toe was kicking ．And the man broke it once．Perhaps he was playing football outside --- with no boots on．”

(1) People had been using that cave for ______．　　　

[　　]

A．half their lives

B．5, 600 years

C．something like 12, 000 years

D．at least 12, 000 years

(2) The word“remains”in Paragraph Two means ______．　　　

[　　]

A．things left by ancient people

B．dead bodies

C．newly-found ancient places

D．caves where ancient people lived

(3) C14 is a natural element ______．　　　

[　　]

A．but it kills quickly

B．and we all have some in us

C．found only in bones

D．but it disappears after 5, 600 years

(4) It is true that only ______ a half life．　　　

[　　]

A．C14 has

B．old bones have

C．radio-active materials have

D．a living thing has

(5) Bones from an animal that died recently would show ______ C14 in them．

[　　]

A．a high level of 

B．a low level of

C．no

D．very little

Answer 1

答案：D;B;B;C;A