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.