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

This answer is irrelevant. It could very well be true but we want to know why the entomologists' effort was wasted.

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

We don't care whether a parasite will control the pest population. We are trying to ascertain why the entomologists' effort was wasted.

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

We don't care about the chances. Totally irrelevant.

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

Bingo.

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

This may seem like a good answer choice but it conflicts with the information in the passage which states that the entomologists confined their search to parasites of the sweet-potato whitefly. This essentially means that the sweet-potato whitefly has parasites.

IMO D

thephoenix nailed it!


When dissecting this problem, Kaplan recommends first identifying the author's conclusion--in this case, that the search has been "a wasted effort." Why has it been a wasted effort? Well, because they've been looking at sweet-potato parasites to kill a silverleaf whitefly.

So what's the gap? What's missing? Well, the author doesn't tell us why we've failed. If pests could cross species, we might well find a decent parasite, one that kills ALL whiteflies! But he is assuming this isn't the case--that no sweet-potato whitefly can help in the annhilation of the silverleaf whitefly.

If we couldn't make this logical deduction, then we can do what thephoenix did and eliminate all the answers. But if we did predict the answer before we got to the answer choices, we'd have no problem zeroing in on (D), the answer that matches our prediction exactly.

gmatnmein2010 wrote: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?

Premise : all the research for the parasite is done thinking it belongs to sweetpotato white fly which
indeed belongs to silverleaf whitefly.

Conclusion : Search for a parasite for Silverleaf Whitefly has been wasted.

In an assumption Question the correct answer choice Must be True and connected to the conclusion for the conclusion to be true. Lets see which answer choice does that for us :

A) All varieties of the sweet-potato whitefly are serious crop pests.
Incorrect : Has no impact on the conclusion. Ruled out.

B) If a crop pest has a parasite, that parasite can always be used to control that pest.
Incorrect : Issue is about SEARCH for a parasite to control the pest. It is not about whether or not that pest can be used to control the pest. Eliminated.

C) The chances of successfully identifying a useful parasite of the new pest have increased since the proper identification of the pest.
Incorrect : What does it matter whether chances have increased or not? We want to know WHY the search for a parasite was wasted. Out of scope.

D) No parasite of the sweet-potato whitefly is also a parasite of the silverleaf whitefly.
Correct : If we negate this answer choice ( A parasite of the sweetpotato whitefly is also a parasite of the silverleaf whitefly) it destroys the argument. Hence this is the correct answer choice.

E) In the last three years, the entomologists found no parasites of the sweet-potato whitefly.
Incorrect : This is kind of extreme statement which says that entomologists found NO parasites in 3 yrs. Eliminated

Hope this helps : )

If we use Kaplan Denial Test to D, the argument falls apart.

Bingo ! [spoiler]D![/spoiler]