Vaishali2004
Choice A - Even if the rate of deposit of salt was not constant. We could still determine the age by assuming that a constant amount being depotited. That way, we would end up determining higher age of earth than in real. But the conclusion mentions "maximum age" So then wouldn't that still be okay?
Let's say that salt deposits HAVE been unusually high in the past century. If you used this data and mistakenly said that the same thing happened in previous centuries, you'd come up with a time estimate that is
too short (as in, you'd think that the water got saltier in a
shorter amount of time than it actually took). This would give you a (faulty) maximum age that is too recent. The
actual maximum age would be older than your estimate.
So, you need to assume that quantities of dissolved salts deposited by rivers in the Earth’s oceans have
not been unusually large during the past hundred years.
That's why (A) is an assumption on which the argument depends.
I hope that helps!