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

A. The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.

Explanation:

In the above given passage, the salty nature of the seas/ oceans are due to the "dissolved salts" carried into them by the rivers. Now, considering the level of salt in the oceans to be zero in the start, then the rivers' gradual deposits make up the salty nature. And this will take hundreds of years to make a whole ocean salty. So, if we think about the salt level of the current oceans and the process it takes, hypothetically, then we can safely assume that the quantities of dissolved salts deposited by the rivers are not that large during the past hundred years. For, if the deposits were in large quantities, the oceans' salt levels will have skyrocketed by now.

Thus, the correct assumption the argument depends on is option A.

Option B is irrelevant, option C too. If salts have leached into the oceans directly, then that would have also happened hundreds of years ago. Option D is also again irrelevant for the passage never mentioned the superiority of the method in estimating the salt levels. Option E is considerable but void in the part of the salts being used for biological activities.

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