Certain fossils, called index fossils, help geologists match rock layers. To be useful as an index fossil, a fossil must be widely distributed and represent a type of organism that existed for a brief time period. Index fossils are useful because they tell the relative ages of the rock layers in which they occur.
I think carbon dioxide then sugars
I believe Option C would be your best answer. It says the best way, so either Option C or Option D.
Hope this helps!
~Autumly