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PREMISE: Rivers carry dissolved salts into oceans.The Earth's rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the oceans to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated.
Which of the following is an assumption on which the argument depends?
A.The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.
B.At any given time, all the Earth's rivers have about the same salt levels.
C.There are salts that leach into the Earth's oceans directly from the ocean floor.
D.There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.
E.None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.
CONCLUSION: Measuring the salt increase for the past 100 years will allow us to determine Earth's age
We're looking for a necessary assumption. In other words, we're looking for information that MUST be true in order to reach our conclusion. So, if we NEGATE each answer choice, the one that destroys the conclusion will be the correct answer.
A) The quantities of dissolved salts deposited by rivers in the Earth's oceans HAVE been unusually large during the past hundred years.
Does this destroy the conclusion that measuring the salt increase for the past 100 years will allow us to determine Earth's age?
YES! The conclusion requires that the salt increase over the past 100 years is REPRESENTATIVE of the salt increase over millions of years. When we NEGATE answer choice A, it says that the 100-year salt increase IS NOT representative.
KEEP A
B) At any given time, all the Earth's rivers DO NOT have about the same salt levels.
Does this destroy the conclusion that measuring the salt increase for the past 100 years will allow us to determine Earth's age?
No. The conclusion is based on the overall amount of salt that reaches the oceans.
ELIMINATE B
C) There are NO salts that leach into the Earth's oceans directly from the ocean floor.
Does this destroy the conclusion that measuring the salt increase for the past 100 years will allow us to determine Earth's age?
No. ELIMINATE C
D) There IS a method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.
Does this destroy the conclusion that measuring the salt increase for the past 100 years will allow us to determine Earth's age?
No. The negated answer choice simply says that a better method exists. This does not mean that the method suggested in the argument will not work? No.
Since the negated answer choice does not destroy the conclusion, we can ELIMINATE D
E) SOME of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.
Does this destroy the conclusion that measuring the salt increase for the past 100 years will allow us to determine Earth's age?
Not necessarily. First of all, the conclusion is based on the NET amount of salt that remains in the oceans, and this negated answer choice does not necessarily ensure that the NET salt accumulation in the ocean over the past 100 years is not representative of the ocean's lifelong salt accumulation. All we know is that SOME salts are used up by biological activity. Since "some" can range from 1 salt molecule to huge amounts of salt, the negated answer choice does not necessarily destroy the conclusion.
ELIMINATE E
Answer: A
Cheers,
Brent
