Bird skeletons are very delicate, and therefore, difficult to be preserved. The bones of the birds are soft. The small animals are very prone to physicochemical as well as biological decay. This is why the fossil record of bird is incomplete and well-preserved fossils of birds are rare to find, except at a few unusual sites.
The answer is D. The sugar belongs to carbohydrates. The lipids is consisted of glycerol and fatty acid. The proteins is consisted of amino acids. The nucleic acids is consisted of nucleotide.
In order:
kingdom, phylum/division, class, order, genus, species
