This mystery rocks! by cyntha schlagel

the drifting rocks are a strange phenomenon still unexplained by science. located in death valley, california, the rocks sit on hot, flat ground. unlike normal rocks, they have trails etched behind them as if they have traveled across the sand. some trails are only a few feet. some trails are over a half a mile long. each trail is as baffling as the next.

the variety of rock movement has baffled scientists for decades. some rocks seem to roll as they move forward. some take unexplainable routes. large ones have traveled past small ones that have stayed still. some scientists suggest that the rocks are pushed by wind. others believe they slide on small amounts of ice or mud. so 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?

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