Compound

A. a living component of an ecosystem; for example organisms, such as plants and animals.

B. a substance made up of two or more different chemical elements combined in a fixed ratio.

C. a chemical substance that cannot be broken down into other substances. The basic particle that constitutes a chemical element is the atom, and chemical elements are distinguished from each other by the number of protons in the nuclei of their atoms

D. a molecule containing a very large number of atoms, such as a protein, nucleic acid, lipid, or carbohydrate