Calculating the age of a stone tool is not easy. The stone does not lend itself to radiometric dating even though it may have been made of volcanic rock. The reason is that the hominid who made it may have picked up a piece that was fifty or a hundred million years older than he was. He may even have made the tool in one place and dropped it in another. Furthermore, tools that have been chipped and banged about may have suffered so much alteration to their surfaces that any date derived from them will contain an unacceptably large degree of error. Therefore one must look at the tool itself and at its geological and fossil associations.

From page 229 of Lucy: The Beginnings of Humankind by Donald C. Johanson and Maitland A. Edey (New York: Simon and Schuster, 1981).

Johanson and Edey (1981) pointed out that hominids may have used rocks that were fifty or a hundred million years older than themselves when making stone tools.

Johanson and Edey (1981) pointed out that it is difficult to pinpoint the age of stone tools because of a variety of factors including the age of the stone used, the transporting of tools, and the degradation of tools from normal usage.

The second answer is correct because it accurately summarizes the main point of the passage using the student writer’s own words. The first choice is off the point, and it borrows language from the original source without citing it – a form of plagiarism.