A student made a model of a river in a sandbox. A large
pile of sand represents soil and bits of rock in the riverbed.
Water pouring from a hose represents river water as it
flows downhill.
One limitation of the model is that it does not represent how living things
affect the flow of a river. Which way of refining the model would best address
this limitation?
A. Planting small houseplants along the slope of the sand pile
B. Pouring the water from a bucket instead of a hose
C. Using a pile of gravel instead of a pile of sand
D. Spreading out the sand so that it looks more like flat ground