My original prediction before seeing the answers was that the rate by which salt was deposited in the last century must be representative of the centuries since the dawn of Earth.
But, since I'm still a careless noob at this point, I broke from my strategy and chose E.
E. None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.
Remember, though, that we're looking only for an accurate estimate
. So we don't need to be so bold (well, extreme) and assume none
of the salts are used up by biological activity. They can. This is an estimate after all, right?
A. The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
Reading A again, I should have stuck with my prediction. For this argument, we must
assume that the salt deposited wasn't unusually large during the last century. Else, we'd undershoot the age of the Earth -- certainly more than by a variance in our estimate of age due to biological activity. A is the correct answer IMO.
GMAT Day: Nov. 19, 2011