Cellular respiration is a process used in cells that converts glucose and oxygen into carbon dioxide and water. Using the image, which letter represents the reactant oxygen?

Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP)

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."