Scientists have found methods to clone organisms. the scientists are not concerned with
a) collecting valid data.
b) having large sample sizes.
c) having controlled experiments within ethical limits.
d) having controlled experiments with only one variable.

c - having controlled experiments within ethical limits.

Explanation:

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.

answer: land bridge theory. land bridge theory held that shifting portions of dry land explained the finding of identical species on different continents. plate tectonic theory eventually eliminated the land bridge theory.