六级冲刺练习：阅读（94）

　　Scientists have found that there are three 13 stages in a mosquitos assault. Stage one is at fifty feet away, when the insect first smells a man or a animal to 14 . Stage two is thought to come into operation about twenty-five feet from the target, when the insect becomes guided by the carbon dioxide breathed out by the intended victim. Stage three is when the mosquito is only a matter of inches from its 15 the warmth and moisture given off by the victim is the final clue.

　　The researchers then 16 how repellents interfere with its three-stage attack. They found repellents act more subtly than by just giving off a nasty smell. A Canadian researcher says that repellents appear to 17 mosquitoes first when it is following the carbon dioxide and second during the final approach, where the warmth and moisture are the insects 18 .

　　Air pervaded by one of the many chemical repellents stops the mosquito reacting to the victims carbon dioxide, and the repellent seems to affect the tiny hairs with which the insect senses moisture in the air. The sensors are blocked so that the 19 does not know whether it is flying through a moist current, or

　　the sensors are made to send the 20 signals.

　　A. examined B. animal C. wrong D. insect

　　E. bite F. Unfortunately G. inventing H. distinct

　　I. prey J. guide K. checked L. definite

　　M. Unnecessarily N. confuse O. discovering

　　11. O 12. F 13. H 14. E 15. I 16. A 17. N 18. J 19. D 20. C