Students want to determine the impact of soil erosion from the development of new housing. They plan to measure the density of a species of algae growing along the bottom of a river that flows through their town. The town is built in a heavily forested area, but a large area of trees along the river was recently clear-cut to allow for the construction of a new housing development, which has caused exposed soil to wash into the river. The river flows north to south through the town and is still forested both up- and downstream of the town. Which of the following would be the best control sample for the study?

a. A sample of water collected just downstream of the northern forest
b. A sample of water collected just downstream of the southern forest
c. A sample of distilled water
d. A sample of water collected just downstream of the housing development