S92-14

Official Solution:


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.


The author of this argument concludes that, because an active chemical in an insecticide becomes inactive after 14 days of air exposure, the death of an insect that lands on a sprayed tree is not due to the ingestion of this insecticide.

The answer must provide evidence against the following chain of events: Insecticide is sprayed, active chemical is exposed to the air, 14 days pass, chemical becomes inactive, insect's death is not due to this pesticide.

Choice D is correct. It provides evidence that the chemical, once sprayed, is not necessarily exposed to the air for fourteen continuous days. If a rainstorm can carry the insecticide into a stream and preserve its activity, it is not until the chemical is re-exposed to the air that its activity resumes. Thus, this chemical could evaporate, rain back down onto the tree, and cause the death of an insect outside of the 14-day window. This weakens the argument.

Choice A is outside the scope of the argument, which is concerned with the effect of this insecticide on an insect.

Choice B is irrelevant; there is no evidence that the amount of insecticide has any effect on how long it remains effective. After 14 days, all the insecticide, no matter how much there is, should become inactive and non-lethal.

Choice C is about an irrelevant group of insects; the conclusion is that insects that land on the tree do not die from ingesting this insecticide; the insect eggs inside the tree are out of the scope of this argument.

Choice E is irrelevant to the conclusion. This choice states that the insecticide might have another lethal effect on insects, causing them to die from malnutrition. However, the argument states explicitly that insects that land after 14 days do not die from ingesting the insecticide. This choice actually strengthens that argument, by offering another reason that the insects might have died on this tree.


Answer: D