The Earth consists of a variety of different organisms. Despite their diversity, however, some similarities on the structural and metabolic levels may be seen. These similarities can be attributed to A. similarities in organisms' habitats and niches. B. similarities among organisms in their proteins and nucleic acids. C. the ability of organisms to readily adapt to sudden environmental changes. D. the ability of all organisms to interbreed and produce new species.

B. similarities among organisms in their proteins and nucleic acids.

Explanation:

Most enzymes, hormones cofactors etc involved in biochemical reactions in animals are all made of proteins. with nearly the same polypeptide chains of amino acids.

Therefore despite the variation in the sequence of the amino acids in these proteins, they still shared the same functional characteristics and features common in all. E. g Enzyme pepsin, made in protein synthesis catalysis proteins to amino acids in the stomach of human. while Proteases also made of protein, catalyzed the breakdown of long polypeptide chains to amino acids in Plants. Thus despite present, and performing similar roles in different organism, these enzymes share the same structural similarities which showed similarities between the organisms where they are found.

The same is applicable to the generation of ATPs (Chemeosmosis) in thylakoid and mitochondria, of plants and animals respectively. These metabolic similarities is due, the processes of gene expression which follow relatively the same mechanisms in virtually all organisms, futher showing the similarities.

In addition the genetic make up in all organisms of the same species shares almost the same gene composition (Nucleic acids), although with some variation. The gene expression as proteins is responsible for productions of major hormones, cellular and tissues developments which although have varied composition in different organism, showed same functional characteristics for similar structural levels observed in different organisms.

These differences can be attributed to "similarities among organisms in their proteins and nucleic acids"

Explanation:

Both proteins and nucleic acids are essentially made up of the same structural components in all living organisms, amino acids in the case of proteins and nitrogenous bases in nucleic acids