ttram wrote:
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 ocean to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimate.
Which of the following is the 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.
We can eliminate B, C and D after reading.
The argument is talking about: We can guess the maximum Earth's age when we calculate dissolved salts from rivers to oceans with the premise is at the first time, the salt level of oceans is zero.
B: Irrelevant because we do not discuss about salt level in rivers. It does not lead us to the conclusion.
C: Irrelevant, too.
D: Although it seems to be a right answer because it talks about a method. However, it is too general, and not assume anything.
E: The first time, I think E is the best answer. However, it is a mistake. First, it is too specific. If none of the salts were used by biological activity, so, how about other activities? (such as physical activity, chemical activity...)? Second, it does not lead us to a conclusion. We cannot conclude that the method is accurate because of the premise E.
A: It supports the premise:
Premise:
- At the first time, the salt level of oceans is zero.
- We calculate the salt level of oceans in the past hundred years, and we can know the Earth's age.
- In a process, the quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been
unusually large during the past hundred years.
- Thus, we can convince that the result is believable.
I hope that you guys can accept my explanation.