When a stone is trimmed by a mason and exposed to the elements, a coat

When a stone is trimmed by a mason and exposed to the elements, a coating of clay and other minerals, called rock varnish gradually accumulates on the freshly trimmed surface. Organic matter trapped beneath the varnish on stones of an Andean monument was found to be over 1000 years old. Since the organic matter must have grown on the stone shortly after it was trimmed, it follows that the monument was built long before the arrival of Europeans in the Americas in 1492.
Which one of the following, if true, most seriously weakens the argument?

(A) Rock varnish itself contains some organic matter.
This doesn't weaken the argument.

(C) The Andean monument bears striking resemblance to monuments found in ancient sites in western Asia.
Irrelevant as no date is mentioned.

(D) The earliest written reference to the Andean monuments dates from 1778.
Irrelevant. Written references do not matter to us.

(E) Rock varnish forms very slowly, if at all, on trimmed stones that stored in a dry, sheltered place.
Doesn't help in weakening the argument.

(B) The reuse of ancient trimmed stones was common in the Andes both before and after 1492.
Correct Answer: Ancient stones were reused to build monuments in the Andes. So these stones could have been cut out off existing ancient structures and used to built the monuments. So although the stone may be old, it does not necessarily mean that the monuments were that old too.

Although I got this right, I would still call for an end to the use of spoilers. Its much better to write an explanation without knowing what the right answer is. The spoiler should be enabled after someone comes up with the right answer. Just my thoughts