Scientists have proposed reviving an extinct species using one of two possible methods. In the first method, sex cells ( sperm or eggs) are obtained from the extinct animal and are used to fertilize(使受孕) the sex cells of a closely related living relative in a laboratory. For example, sperm from a woolly mammoth could be used to fertilize an egg from a modern-day elephant. The fertilized egg would then be placed in the womb of a live female elephant where it would live and grow until it is ready to be born.

The second method involves a type of cloning. In cloning, the DNA of one individual replaces the DNA of another. In the woolly mammoth example, scientists could inject DNA from a mammoth into an egg cell from an elephant. The cloned egg cell would then be placed into a living elephant and allowed to develop in the same way as a fertilized egg.

Many difficulties remain before it will be possible to revive an extinct species by either method. In fact, some scientists believe that because of all the problems, species revival will never happen. One of the major challenges is to obtain enough high-quality DNA from an extinct species to conduct an experiment. While it is theoretically possible to preserve genetic material for thousands of years under ideal conditions, these conditions are very hard to find in real life. For example, researchers have obtained a number of samples mammoth DNA, but none have been usable. And the cloning procedure presents its own problems. Scientists have been able to clone only a few species of animals, and most cloned creatures are short-lived and frail.

And there is a final, ethical consideration. Even if we learn how to reproduce an example of an extinct species, that individual could never have a normal life. Its natural environment is most likely gone, and it would have no parents to show it how to behave as a member of its species. So it would remain a curiosity, and probably live out its life in a zoo. People question whether it would be ethical to revive one of nature’s creatures for such a purpose.

50. What is the main idea of the passage?

A. Reviving extinct animals remains to be seen.

B. Reviving extinct animals is only science fiction.

C. Reviving extinct animals will surely happen in the future.

D. Reviving extinct animals is supported by all.

51. What was the main cause for the extinction of the thylacine, the moa, the mammoth and the bucardo?

A. changing weather B. lack of food

C. human behavior D. disappearing habitat

52. Which of the following is a method for reviving extinct species?

A. fertilization of the egg and the sperm of an extinct one.

B. the cloning of an extinct species.

C. finding an egg and warming it under right conditions.

D. injecting DNA into an existing species.

53. What is true about cloned animals?

A. They are usually very healthy.

B. They are often weak in health.

C. They look the same as others.

D. They are well protected by scientists.

54. What can be inferred from the passage?

A. We should revive extinct species as soon as possible.

B. We should return many animal populations to the wild.

C. Reviving extinct species is a highly difficult process.

D. Reviving extinct species is good for both nature and humans.