Standard archaeological techniques make it possible to determine the age of anything containing vegetable matter, but only if the object is free of minerals containing carbon. Prehistoric artists painted on limestone with pigments composed of vegetable matter, but it is impossible to collect samples of this prehistoric paint without removing limestone, a mineral containing carbon, with the paint. Ttherefore, it is not possible to determine the age of prehistoric paintings on limestone using standard archaeological techniques. Which one of the following, if true, most seriously weakens the argument? ['The age of the limestone itself can be determined from samples that contain no vegetable-based paint.', 'The proportion of carbon to other elements in limestone is the same in all samples of limestone.', 'Prehistoric artists did not use anything other than vegetable matter to make their paints.', 'Laboratory procedures exist that can remove all the limestone from a sample of prehistoric paint on limestone.']