Describe one way to correlate similar rock layers

These fossils are very useful for us because we can use them to date the layers of rock that they are found in. Fossils that can be used in this way are called index fossils, and rock layers with the same index fossils in them can be correlated.     Not my answer, link here for full

http://peter-mulroy.squarespace.com/correlating-rock-layers/

c should be the answer

"it built up in the atmosphere"

i think thats it, idk, i tried looking for this exact q& a for grad. and idk if i got this question right or sorry if im wrong