Alice is studying how human activities impact ecosystem biodiversity in her college class. Her latest assignment is to design and test a model that will demonstrate a method to reduce the environmental impact of a specific human activity.

Alice notices that a local farm is situated on the edge of a river. She then confirms that chemical fertilizers are entering the river ecosystem through surface runoff, potentially harming aquatic wildlife and reducing biodiversity.

farm situated near a river
Alice decides to build a functional scale model to see if it can reduce the amount of fertilizer entering the river. After explaining her assignment to the farm's owner, she is granted permission to design and test her model at the site.

Which design would best fulfill Alice's assignment?

A.
mark off a 10-foot-wide strip of land between the farm and the river and transplant closely-spaced perennial vegetation there
B.
dig a six-inch-wide channel from the farm to the river in order to concentrate the surface runoff and limit the exposed area
C.
build a one-foot-tall retaining wall out of sandbags around the farm to prevent surface runoff from reaching the river
D.
section off a 100-square-foot area of the farm and ask the farmer to use twice the normal amount of fertilizer there