Passage Three Questions 31 to 35 are based on the following passage. One of the most exciting races ever run is now in progress between doctors fighting malaria(疟疾)and mosquitoes. According to the most recent counts 225 million people a year suffer attacks of malaria and more than two million die. Public health workers around the world are doing all t11ev can to destroy malaria before the mosquitoes that pass on the disease become resistant to the poisons now used against them. It's a race against time and against difficulties，with millions of lives in danger and the chances of winning not in man's favour. Malaria，it is true，has been practically wiped out in thirteen countries. including the United States，and is under attack in many others. But it is equally true that in some parts of the world certain types of malaria-carrying mosquitoes have already learned to resist some of the sprays that formerly killed them. Other types of mosquitoes are not killed as quickly by present sprays as they once were. The World Health Organization is helping national governments to get rid of malaria before resistance among the mosquito population becomes so great that new poisons will have to be found to replace those in use at present. Most of the countries in the world have started，or are planning，campaigns against mosquitoes. If the race against resistance is won by man，it is possible that ten years from now dais old evil will have disappeared completely from the America，perhaps from the world. Malaria has been successfully got. rid of in______.