The profusion of skin colors and physical characteristics of different humans is due to .
a. the development of pure races
b. the differences in climate and other living conditions humans encountered as they populated the globe
c. biology rather than evolution
d. the emergence of dominant groups in the last three centuries

poor water quality affects most estuarine organisms, including commercially important fish and shellfish. the pollutants that have the greatest impact on the health of estuaries include toxic substances like chemicals and heavy metals, nutrient pollution (or eutrophication), and pathogens such as bacteria or viruses.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.

true skin; another name for the dermis. lies directly beneath the epidermis and is often referred to as the true skin. outermost layer of epidermis, consisting of dead cells.