题目选项分析

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?

【选项】The quantities of dissolved salts deposited by rivers in the Earth`s oceans have not been unusually large during the past hundred years