Why were the Smith and Cuvier discoveries that similar groups of fossils in distant rock layers could be correlated such a major breakthrough in geology?

A.
It disproved the principle of superposition.

B.
It proved igneous rocks held more information than sedimentary rocks.

C.
Fossils could now be used to identify rock layers.

D.
Fossils were no longer needed to study the history of Earth.

Question

Why were the Smith and Cuvier discoveries that similar groups of fossils in distant rock layers could be correlated such a major breakthrough in geology?
 
 A.
It disproved the principle of superposition.
 
 B.
It proved igneous rocks held more information than sedimentary rocks.
 
 C.
Fossils could now be used to identify rock layers.
 
 D.
Fossils were no longer needed to study the history of Earth.