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Some plant pathologists have turned to genetic engineering to devise new ways of responding to botanical diseases. Plant pathologist William Powell, for instance, teamed up with forest biologist Chuck Maynard to restore North America's population of American chestnut trees, which was decimated in the early 1900s by a fungus known as chestnut blight. A fungus, C. parasitica, contributes to the growth of cankers on the trees' trunks; these cankers, in turn, prevent a tree from transporting water from their roots to their branches and leaves. To render American chestnut trees resistant to the blight, Powell and Maynard experimented with altering the genetic makeup of the trees. The resultant genetically engineered trees are called Darling 4 American chestnut trees, and during the first 15 weeks after infection with chestnut blight, they grow cankers that are much smaller than those that grow on American chestnut trees.

A.NO CHANGE
B.trees
C.that tree
D.it

答案和解析

答案：B

解析：