Fossils of ocean clams are found in the rocks of the Appalachian Mountains. What do these fossils most likely indicate?

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Fossils of ocean clams are found in the rocks of the Appalachian
Mountains. What
do these fossils most likely indicate?

A. The area that is now the mountains was once under the ocean.

B. The mountains once had a more rainy climate.

C. The rock layer in which these fossils were found is composed of igneous
rock.

D. The rock layer in which these fossils were found is composed of
metamorphic rock.