Question ID:PT31 S2 Q2 Passage:For the last three years, entomologists have been searching for a parasite to help control a whitefly that has recently become a serious crop pest.  Believing this new pest to be a variety of sweet-potato whitefly, the entomologists confined their search to parasites of the sweet-potato whitefly.  Genetic research now shows the new pest to be a distinct species, the silverleaf whitefly.  Therefore, the search for a parasite has so far been wasted effort. Stem:Which one of the following is an assumption on which the argument relies? Correct Answer Choice:DChoice A:All varieties of the sweet-potato whitefly are serious crop pests. Choice B:If a crop pest has a parasite, that parasite can always be used to control that pest. Choice C:The chances of successfully identifying a useful parasite of the new pest have increased since the proper identification of the pest. Choice D:No parasite of the sweet-potato whitefly is also a parasite of the silverleaf whitefly. Choice E:In the last three years, the entomologists found no parasites of the sweet-potato whitefly.