I think the answer is B, because only plant cells contain a cell wall, while the other answers contain an organelle that can be found in multiple cell types.
The reason is that something changed that caused them to be unsuccessful in their environment.
Sorry if it is not right.
I think A is the youngest layer, because of the new sediments that fell upon the animal.
1) pathogen enters body and releases chemoattractants
2) a phagocyte/macrophage is attracted by these chemicals
3) the phagocyte engulfs the pathogen using its pseudopodia
4) the membranes of both organisms fuse forming a phagosome
5) lysosomers in the phogocyte fuse with the phagosome, forming a phagolysosome
6) these enzymes digest the pahogen (hydrolyse the bonds)
7) the antigens of the pathogen are placed on the surface of the phagocyte making an APC (antigen presenting cell)
