Answer:
The correct answer is option c. "human population".
Step-by-step explanation:
A lot of studies suggest that an increase of human population in most cities results in an increase on the amount of sewage on nearby rivers. If a large number of sewage overflow sites are found in the Susquehanna River it is very likely that is a direct result of an increase in human population. More people living in the cities of the states of New York, Pennsylvania, and Maryland would directly result in an increase in the sewage overflow of the river.