Bollworms are a traditional threat to cotton crops．Young ones feed on the boll, the part of the
cotton plant that contains the seeds．
A bacterium known as Bt is able to kill bollworms and some other pests．Bt is short form for Bacillus thuringiensis．It was discovered in the early twentieth century．Farmers began using Bt as a natural pesticide（杀虫剂）．
Then，in the 1990s，researchers found a way to grow cotton plants that contain a Bt gene．The genetically modified cotton plant produces toxins that kill bollworms．Farmers around the world are now growing an estimated fourteen million hectares of Bt cotton．
Cotton bollworms also attack other crops．But scientists in China recently reported that Bt cotton may help suppress bollworm in other crops growing nearby．
The study involved crops grown in six provinces of northern China between 1992 and 2008.The study area contained three million hectares of cotton and twenty-two million hectares of corn，peanuts，soybeans and vegetables．
The researchers say the study suggested that Bt cotton not only controls bollworm on the transgenic cotton，but also may reduce its presence on other crops．And that reduction，they say，may decrease the need for insecticide use in general．
But the researchers also pointed out concerns．They say bollworms could develop resistance to the cotton plants designed to resist them．And they noted that insects called mirids have become “key pests” of cotton in China．They said this is because of a decrease in pesticide use made possible by Bt cotton．
The researchers say they do not believe that Bt cotton alone can solve all insect problems．Instead，they say farmers in China should consider it just one part of pest management systems．
44．What is Bt in this passage?
A．It's a kind of hi-tech invention．  B．It's a natural bacterium．
C．It's a kind of pest．                 D．It's a man-made pesticide．
45．Bt cotton has all the advantages EXCEPT that ________．
A．it resists bollworms
B．it does good to other crops growing around
C．it reduces the use of insecticide
D．it helps to develop new insects
46．The underlined word “ones” in the first paragraph refers to ________．
A．bollworms         B．cotton crops        C．cotton seeds        D．pests
47．From the passage，we can infer ________．
A．farmers around the world are now growing a large area of Bt cotton
B．researchers are probably now searching for new ways to solve other cotton problems
C．bollworms could not exist in Bt cotton any longer
D．Chinese scientists found the way to grow Bt cotton

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