The method is purely based on the observations from the past 100 years, so we need to something which points out the flaw in this method.
(A) The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years. CORRECT. this says the rate of salt deposited in the last 100 years is unusually high. This clearly weakens the argument.
(B) At any given time, all the Earth’s rivers have about the same salt levels. irrelevant
(C) There are salts that leach into the Earth’s oceans directly from the ocean floor. if salt leaches into the oceans directly, it would have done so in the past 100 years as well. so it doesnt point to a flaw in the proposed method.
(D) There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans.
Ignore it
(E) None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.
if oceans use salt for biological activities. it would have done so in the past 100 years as well. so it doesnt point to a flaw in the proposed method
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