The next question refers to This Mystery Rocks! by Cynthia Schlagel. The sentences have been numbered to help you identify them more easily.

This Mystery Rocks! By Cyntha Schlagel

1The Drifting Rocks are a strange phenomenon still unexplained by science. 2Located in Death Valley, California, the rocks sit on hot, flat ground. 3Unlike normal rocks, they have trails etched behind them as if they have traveled across the sand. 4Some trails are only a few feet. 5Some trails are over a half a mile long. 6Each trail is as baffling as the next.

7The variety of rock movement has baffled scientists for decades. 8Some rocks seem to roll as they move forward. 9Some take unexplainable routes. 10Large ones have traveled past small ones that have stayed still. 11Some scientists suggest that the rocks are pushed by wind. 12Others believe they slide on small amounts of ice or mud. 13So far, research has not confirmed any theory.

Read the following sentence from This Mystery Rocks!:

Unlike normal rocks, they have trails etched behind them as if they have traveled across the sand.

How does the sentence contribute to the main idea of paragraph one?

It reveals why the drifting rocks are strange.
It explains where the rocks may be found.
It identifies the persons who are named on the rocks.
It offers a solution for why the rocks move.