Check out this challenging question from the GMAT paper tests offered by GMAC:

Potato cyst nematodes are a pest of potato crops. The nematodes can lie dormant for several years in their cysts, which are protective capsules, and do not emerge except in the presence of chemicals emitted by potato roots. A company that has identified the relevant chemicals is planning to market them to potato farmers to spread on their fields when no potatoes are planted; any nematodes that emerge will soon starve to death.

Which of the following, if true, best supports the claim that the company’s plan will be successful?

(A) Nematodes that have emerged from their cysts can be killed by ordinary pesticides.

(B) The only part of a potato plant that a nematode eats is the roots.

(C) Some bacteria commonly present in the roots of potatoes digest the chemicals that cause the nematodes to emerge from their cysts.

(D) Trials have shown that spreading even minute quantities of the chemicals on potato fields caused nine-tenths of the nematodes present to emerge from their cysts.

(E) The chemicals that cause the nematodes to emerge from their cysts are not emitted all the time the potato plant is growing.

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Conclusion: Nematodes = pest; spreading chemicals on fields = starving nematodes

Evidence: They emerge b/c of potato root chemicals; company going to market chemicals to potato farmers

Assumptions: -Nematodes do something bad to the potato; Nematodes will ONLY damage potatoes; the chemicals will not cause any secondary damage to potatoes

Question Rephrase: What will make the plan STRONGER?

Prediction: Proof the nematodes will die (chemicals are effective), that this will effectively protect potatoes with no secondary damage

(A) – Weakens. Makes the chemicals less valuable.

(B) – Weakens. Makes the nematode less dangerous.

(C) – Weakens. Shows chemicals are GOOD for potatoes.

(D) – Strengthens. Shows the chemicals are effective!

(E) – Weakens. This does not relate since the chemicals will be put on fields with no potatoes planted yet.

The correct answer is (D).