The student is trying to see which substance (disinfectant 1, 2, 3, or the distilled water) can disinfect the most.
Its the Eccentricity of the earth. (shape of Earth's Orbit around the sun, whether it is more circular or ovular.)
As you go up in an ecosystem the bigger animals have less energy. While the organisms down at the bottom like plants that create their own energy have greater amounts. This is because energy is rated doing everyday things. As energy travels up the food pyramid, it looses more and more energy. This is knower as the 10% rule. In which every time you go up 10% of energy is lost.