Some plant pathologists have turned to genetic engineering to devise new ways of respondingto botanical diseases. Plant pathologist William Powell, for instance, teamed up with forestbiologist 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. Torender American chestnut trees resistant to the blight, Powell and Maynard experimented withaltering the genetic makeup of the trees. The resultant genetically engineered trees are calledDarling 4 American chestnut trees, and during the first 15 weeks after infection with chestnutblight, they grow cankers that are much smaller than those that grow on American chestnuttrees.
Choice B is the best answer because it maintains agreement between the plural noun "trees"and the plural pronoun "their." This choice is also consistent with the first part of thesentence's use of plural trees.