Smith and cuvier discovered that similar groups of fossils in distant rock layers could be correlated. why was this such a major breakthrough in geology

Below choices should accompanied by question above:

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.

I think the answer should be B.  

Cpopulation density