Calcified bones marked with scratches other than tooth marks were discovered in Harrapa excavations, near some stone tools. This was used as evidence that the residents used stone tools because when bones were scratched using similar tools, the resulting scratches resembled the scratches found on the fossils. Therefore, stone tools had probably been used on the animal bones that became calcified.

Which of the following, if true, most seriously undermines the argument?

From the fact that bones scratched with similar tools produced scratches similar to those found on fossils, the author argues that stone tools had been used on bones that become fossilized. To weaken it, we would choose an answer choice that would attack the only evidence given in support of the conclusion. We have been asked to locate that choice which seriously weakens this argument. Choice (2) says that periodic earth movements can produce similar scratches as found on the fossils i.e. the scratches necessarily were not made by the stone tools. Therefore it is the answer.