GMAT Critical Reasoning

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Source: Knewton

Level: 4

When a certain insecticide is sprayed onto a given tree, the active chemical in this insecticide becomes inactive, and non-lethal to insects, after being exposed to the air for 14 days. This chemical becomes active as soon as a tree is sprayed and is no longer considered active as soon as it is ingested by an insect, which dies immediately upon this ingestion. Therefore, the large number of insects that have been observed to die after landing on a tree more than 14 days after this tree has been sprayed must not have died from ingesting the insecticide.

Which of the following, if true, most seriously weakens the conclusion above?

  • A Some laws about insecticide use do not ban the use of several insecticides at a given time, as long as these insecticides are not sprayed simultaneously.
  • B Machines, which are often used to spray an insecticide the first time this insecticide is used in a given region, often spray more insecticide than is necessary to kill the insects on any given tree.
  • C The insecticide can slowly penetrate the bark of a tree and poison any insect eggs lain inside, preventing these eggs from hatching for more than 14 days after the initial insecticide spraying.
  • D Within 14 days after a tree is sprayed, rainstorms may carry the insecticide into a nearby stream, preserving the activity of its chemicals until it evaporates into the atmosphere and precipitates down with the rain.
  • E This insecticide can cause leaf dehydration, which sucks the nutrients out of the leaves, causing insects that depend on the tree for nourishment to die.

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