A、To make African bees less active. B、To increase the number of bees in Brazil. C、To make Brazilian bees more easy-going. D、To i

Scientific experiments can sometimes go wrong and when they do, the results may range from the disastrous to the troubling. One such experiment took place in South America about fifty years ago. Whether its final consequences will cause serious damage or nothing more than a small trouble still remains to be seen.
    The story began in 1956 when an American scientist working in Brazil decided to solve the problem of increasing the productivity of that country’s bees. He imported a very active type of African bee from Tanzania and mated it with the more easy-going native variety to produce a new kind of bees. The new bees worked harder and produced twice as much honey. It seemed that Professor Kerr, for that was the scientist’s name, had a total success on his hands.
    Then things began to go wrong. For some reason as yet unseen, but perhaps as a result of something in their environment, the new bees began to develop extremely attacking personalities. They became bad-tempered and easy to be angry, attacked the native bees and drove them from their living places.
    But worse was to follow. Having taken over the countryside, the new bees, with their dangerous stings, began to attack its neighbors’ cats, dogs, horses, chickens and finally man himself. A long period of terror began that has so far killed a great number of animals and about 150 human beings.
    This would have been bad enough if the bees had stayed in Brazil. But now they are on the move, heading northwards in countless millions towards Central and North America, and moving at the alarming speed of 200 miles a year. The countries that lie in their path are naturally worried because it looks as if nothing can be done to stop them.
Questions 33 to 35 are based on the passage you have just heard.
33. What do we learn about the results of the South American experiment?
34. For what purpose was the experiment mentioned in this passage designed?
35. Which may be the cause of the new bees’ attacking personalities?