The question is asking for assumption.

Just read the prompt

Regards

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The question states that by calculating the increase in salt levels in the oceans over the past hundred years, the maximum age of the Earth's oceans can be accurately estimated.

It assumes that a river carries constant amount of salt each year into the ocean and the salt level of the ocean increases proportionately each year.

This is crucial to our assumption that the amount of salt carried into the ocean remains same each year.

OA (A) states that The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years. This goes in sync with our predicted assumption, so mark it for later after reviewing other answer choices.

(B) At any given time, all the Earth’s rivers have about the same salt levels. - Okay, sounds great. But does it mean they carry the same amount of salt to the ocean, or they encounter certain kind of sands/runoffs in their path which might increase their salt content. Unsure. So, discard it.

(C) There are salts that leach into the Earth’s oceans directly from the ocean floor. - They may have leaching from before 100s of years as well. Discard.

(D) There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans. - Irrelevant & not stated anywhere. Discard.

(E) None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans. - Even if they were, this phenomenon might have been happening from before 100s of years, maintaining the overall salt content of the ocean at a certain level. Either way, used or not, doesn't impact how the conclusion for our question was reached. Discard.