The embryonic endoderm<span> develops into the interior linings of two tubes in the body, the digestive and respiratory tube. the </span>lining<span> of the follicles of the thyroid gland and the</span>epithelial<span> component of the thymus (i.e. thymic </span>epithelial<span> cells). Liver and pancreas cells are believed to </span>derive<span> from a common precursor.</span>
I'd say A: tidal waves. B, C, and D don't really seem to fit for me.
I agree because an ecosystem and a host are basically the same thing.
