阅读理解。 The doctor pointed to some bones on his desk. 'They are a man's foot bones,' he said. 'They were found inside the cave, and I have been examining them. The C14 test shows that they are about 12,000 years old.' 'What is 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 and carbon is found in al l 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 carb on into their bodies from the air. It stays in their remains.' 'Well, there are two kinds of carbon, called carbon 12(C12) and carbon 14 (C14). Both are taken in together, but in living things C12 is one trillion (1012) times more plentiful (大量的) than C14. The two kinds are different in one important aspect.' 'C14 is radioactive:it gives out rays of energy at a fixed frequency (频率). As it does so, it changes to 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 nearly 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 broken. Perhaps the man was playing football outside-with no boots on.' 1. 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 2. Carbon 14 is a natural element (元素) ______. 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 3. It is true that only ______. A. C14 has a half-life B. old bones have a half-life C. radioactive materials have a half-life D. a living thing has a half-life