Passage Four

　　Scientists are hoping to eliminate malaria(疟疾)by developing a genetically modified mosquito that cannot transmit the disease. Malaria has long troubled the populations of South America, Africa, and Asia, where mosquito bites infect up to 500 million people a year with this serious and sometimes fatal parasitic blood disease. For generations, scientists have been trying to eliminate malaria by developing new drugs and using pesticides(杀虫剂)to wipe out local mosquito populations. But these measure aren’t working — and some scientists, like Greg Lanzaro, say that because of drug resistance and population changes, malaria is actually more prevalent now than it was 20 years ago. Lanzaro says he has a better way to stop the spread of malaria: genetically modifying mosquitoes so they are unable to carry the disease.

　　Lanzaro and his colleagues are planning a multi-year project to produce malaria-resistant mosquitoes — and he thinks they can do it within five years. “We can get foreign genes into mosquitoes and they go where they’re supposed to go,” Lanzaro says, pointing out that scientists have already succeeded in genetically engineering mosquitoes that cannot transmit malaria to birds and mice. And, he says, scientists are quickly making progress on genes that block transmission of the disease to humans as well.

　　The most difficult part scientifically, Lanzaro says, is figuring out how to get the lab-engineered mosquitoes to spread their genes into natural populations. After all, he points out, it’s useless to engineer mosquitoes in the lab that can’t transmit malaria when there are millions out in the wild that can. To solve this problem, Lanzaro wants to load up a mobile piece of DNA with the malaria-resistant gene, and then insert it into a group of mosquito embryos. The malaria-resistant gene would be integrated directly into the mosquitoes’ DNA, making it impossible for those mosquitoes to transmit the parasite that causes malaria.In this way a small group of lab-raised mosquitoes could be released into the wild, and by interbreeding with wild mosquitoes, eventually transmit the beneficial gene to the entire population.

　　46.One reason for malaria to be widespread now is that .

　　A.more people have moved to malaria-infected areas

　　B.mosquitoes have become resistant to pesticides

　　C.genetically modified mosquitoes still transmit the disease

　　D.mosquitoes bite as many as 500 million people a year

　　47.Lanzaro is hopeful that in a few years man can .

　　A.start to eliminate malaria

　　B.cure parasitic blood diseases

　　C.prevent mice from transmitting parasites

　　D.acquire immunity against malaria

　　48.Lanzaro is confident that scientists can block transmission of malaria to humans because

　　A.natural mosquito populations do not change

　　B.scientists have succeeded with birds and mice

　　C.foreign genes always go where they are required

　　D.lab-raised mosquitoes will not be resistant to drugs

　　49.What is the most difficult part of Lanzaro’s project?

　　A.Spreading malaria-resistant genes into natural mosquito populations.

　　B.Raising malaria-resistant mosquitoes.

　　C.Making genes that block the transmission of malaria.

　　D.Identifying malaria-resistant genes.

　　50.According to the passage, a fundamental way to wipe out malaria is to .

　　A.develop new malaria-resistant drugs

　　B.produce effective pesticides to kill mosquitoes

　　C.change the genetic makeup of mosquitoes

　　D.remove people from malaria-infected areas

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