Passage Summary: Scientists use a method to date old plants, but it fails if there's any carbon-heavy rock mixed in. Since you can't peel paint off a limestone wall without taking some rock with it, the author says we can't date those paintings.

Conclusion: Standard archaeological dating techniques cannot be used to determine the age of prehistoric paintings on limestone.

Reasoning: These techniques only work on vegetable matter if it is free of carbon-containing minerals, and it is impossible to sample paint from limestone without also picking up pieces of the limestone itself.

Analysis: The author assumes that because the initial sample is 'contaminated' with limestone, the dating process is doomed. To weaken this, we need a way to bypass that contamination. If there is a way to separate the vegetable matter from the limestone minerals after the sample is taken but before the test is performed, the author's conclusion would be proven wrong. Look for an answer that suggests the 'standard techniques' include a way to isolate the paint from the mineral impurities.