20. A. on　　　　　 　 B. in　　　　　　　 　 C. by　　　　　　 　　 D. with

(6)

Scientists in the United States are using teat-tube baby techniques to try to save wild animals ____1_____ extinction(灭绝).

The test-tube technique, or in-vitro fertilization, ____2_____ involves fertilizing eggs(使卵子受精)in the laboratory and then ____3___ them inside the female, became famous when human babies were conceived(怀孕)in this way. The technique is now helping another mammal---the tiger. __4____ the process is successful, it could prevent many species dying ____5_____ . At Minnesota Zoo, in the US, scientists have taken semen from a male tiger and eggs from a female tiger, and ___6_____ them together in the ____7_____. These men fertilizes the eggs and, after a period of about 36 hours, the new embryos(胚胎)can be implanted in the uterus(子宫)of a tigress. This technique ____8_____ the baby tiger to develop naturally ____9_____ the uterus.

A big ____10_____ of this method is that the female tiger does not even have to be the same ___11____ as the embryo. The zoo team found ____12_____ could use a Siberian tiger as a surrogate mother(替身母亲)for Bengali tiger embryos. The test-tube tiger technique could　 ____13_____ in a bank of embryos of ____14_____ animals. An embryo store would help ____15__ a healthy genetic mixture within _16_____ groups of animals.

Many species today are threatened by hunting, or by the ____17_____ of their habitat(居住环境)and food chains. Threatened animals are often _　 18　 _____ in numbers and left in isolated groups. Away from other herds or family groups of their own kind, they are ____59_____ to breed with close relatives(近亲繁殖)and this produces many more unhealthy off-spring (后代)than ____20_____ normally occur.

19. A. foods　　　　 　 B. grains　　　　　 　 C. meats　　　　　 　　 D. fruits

18. A. by　　　　　 　 B. to　　　　　　　 　　 C. as　　　　　　　 　　 D. for

17. A. related　　　　 　 B. concerned　　　 　　 C. charged　　　　 　　　 D. decided

16. A. grades　　　　 B. warnings　　　　 　　 C. cautions　　　　 　　 D. tricks

15. A. caught　　　　 　 B. shared　　　　　 　　 C. divided　　　　 　　 D. separated

14. A. believed　　　 B. accepted　　　 　　　 C. ensured　　　　 　　 D. found

13. A. nationally　　　 B. locally　　　　　 　　 C. intentionally　　 　　 D. purposefully

12. A. If　　　　　　 　 B. Although　　　 　　 C. Unless　　　　　 　 D. Because

11. A. basic　　　　 　　 B. proper　　　　 　　C. least　　　　　　 　　 D. most