An observer noted an increase in the population of mallard ducks in a local river and a decrease in pollution levels. He concluded that the increase in the mallard population was a direct result of decreasing levels of toxic chemicals in the river. What is a more scientific explanation for this increase?
A. The amount of available food in the area increased.
B. The number of mallard predators in the area increased.
C. The number of parasites that live on mallards increased.
D. Emigration of mallards from the area increased.

Question

An observer noted an increase in the population of mallard ducks in a local river and a decrease in pollution levels. He concluded that the increase in the mallard population was a direct result of decreasing levels of toxic chemicals in the river. What is a more scientific explanation for this increase?
A.	The amount of available food in the area increased.	
B.	The number of mallard predators in the area increased.	
C.	The number of parasites that live on mallards increased.	
D.	Emigration of mallards from the area increased.