请教大全Section26-Section 3-19实在看不懂，大家帮帮忙吧

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.

(B) The reuse of ancient trimmed stones was common in the Andes both before and after 1492.

(C) The Andean monument bears striking resemblance to monuments found in ancient sites in western Asia.

(D) The earliest written reference to the Andean monuments dates from 1778.（B）

(E) Rock varnish forms very slowly, if at all, on trimmed stones that stored in a dry, sheltered place.

说是organic东西在varnish下面，那为什么B对呢？被resue了又不会影响Beneath在varnish下面的organic?