Earth science: fossils question pls help​

Layers A and B show cratures that would most likely be found in the ocean (they have fins, etc.) then the next layer has no animals and after that the animals beofre are no longer seen. This must mean that the creatures went exticnt due to an envoirmental issues, in this case its a volcanic explosion (as seen in the key in the corner were the shaded area is indicated as volacanic ash). In layer D, we see a plant, this shows us that the envoirment is now land. After that is another layer of volcanic ash and then F. were we see the previous plants and animlas with the inclusion of 'bird'. I think this means that the envoirment has more greenier wether from the nutrients rich soil (due to the volcanic ash) or another factor.

Explanation:

i am going to guess that it is co2.

the correct answer is b