Swagatalakshmi wrote:
The popular notion that a tree's age can be determined by counting the number of internal rings in its trunk is generally true. However, to help regulate the internal temperature of the tree, the outermost layers of wood of the Brazilian ash often peel away when the temperature exceeds 95 degrees Fahrenheit, leaving the tree with fewer rings than it would otherwise have. So only if the temperature in the Brazilian ash's environment never exceeded 95 degrees Fahrenheit, its rings would be a reliable measure of the tree's age.
Which of the following is an assumption on which the argument above depends?
(A) The growth of new rings in a tree is not a function of levels of precipitation.
(B) Only the Brazilian ash loses rings because of excessive heat.
(C) Only one day of temperatures above 95 degrees Fahrenheit is needed to cause the Brazilian ash to lose a ring.
(D) The internal rings of all trees are of uniform thickness.
(E) The number of rings that will be lost when the temperature exceeds 95 degrees Fahrenheit is not predictable
OFFICIAL EXPLANATION
The author concludes that one will only be able to determine the age of a Brazilian ash by counting its rings if the temperature in the tree's environment never exceeds 95 degrees Fahrenheit. The author bases this conclusion on the fact that the tree loses rings when the temperature exceeds that level. However, if the number of rings lost by a Brazilian ash at high temperatures could be predicted, then it might be possible to determine the age of a tree even if the temperature exceeds 95 degrees. The author assumes that it is not possible to predict the number of rings lost when the temperature exceeds 95 degrees Fahrenheit.
(A) The argument says nothing about precipitation. This answer choice is out of scope since it would require a number of other assumptions to make it relevant to the argument's conclusion.
(B) Whether other trees share this feature is irrelevant; the argument focuses only on the Brazilian ash.
(C) This choice says that one day of temperatures above 95 degrees = one ring lost. If this is true, then we might actually be able to predict the number of rings lost (if we also know on how many days the temperature exceeded 95 degrees). This hurts the author’s argument; therefore, it cannot be an assumption on which the author depends.
(D) The thickness of the rings is irrelevant.
(E) CORRECT. The conclusion is that the rings will be a reliable measure only if the temperature never exceeds 95 degrees. This is true only if there is no way to predict how many rings would be lost when the temperature does exceed 95 degrees. (If it were possible to predict this, one might be able to assess the age of a tree using its rings even if the temperature had exceeded 95 degrees.)