For the last three years, entomologists have been searching for a para

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.

Which one of the following is an assumption on which the argument relies?

A) All varieties of the sweet-potato whitefly are serious crop pests.

B) If a crop pest has a parasite, that parasite can always be used to control that pest.

C) The chances of successfully identifying a useful parasite of the new pest have increased since the proper identification of the pest.

D) No parasite of the sweet-potato whitefly is also a parasite of the silverleaf whitefly.

E) In the last three years, the entomologists found no parasites of the sweet-potato whitefly.

Source: LSAT