The basis of rating the essay with an A are the "clear focus on the hydrologic cycle, photosynthesis, and solar energy and the source of energy for both the stream and its ecosystem."
The questions were based on the statement "Water running down a mountainside erodes its channel and carries with it considerable materials". And the questions were: "What is the basic source of energy used by the water to do this work? How is the energy used by water to do this work related to the energy used by life in the stream ecosystem?
"The process begins with the hydrologic cycle. The sun radiates down and forces evaporation. This gaseous water condenses and forms rain or snow which precipitates back to earth. If the precipitation falls on a mountain, it will eventually run down the hillside and erode its channel. (Some water will evaporate without running down the hill.) The energy used by water to do its work relates directly to the energy used by life in the stream system. The sun is an energy input. It is the source of energy for stream life just as it is the source of energy for the water. Through photosynthesis, the energy absorbed by the stream is used by higher and higher trophic levels. So the sun is the energy source for both running water and the life in the stream. It all starts with solar energy" (Reid 599-600).