Fossil fuels are notoriously dirty sources of energy, and their pollution is often expensive and/or difficult to control.

U-235 differs from U-238 in that U-235 is able to produce a fission chain reaction. U-238 is the most common isotope of uranium, with the U-235 isotope constituting less than 1% of natural sources. Uranium enrichment is the process by which U-235 is separated out, and it is this product that is most commonly used to manufacture nuclear weapons. Pu-239 is produced from U-238 and can be used for nuclear weapons as well, but Pu-239 is not an answer choice.

Producing nuclear energy is a lot cleaner than most people realize, and while it is expensive (but still profitable), it represents a legitimate option for producing cleaner energy. The main drawback is that the resulting "spent fuel" has to be secured and stored, because it cannot be safely disposed of without significant repercussions.

Dams can be huge boons to local economies, and are a landmark of human engineering and achievement; however, they do carry serious consequences to the natural environment. They lead to sediment buildup, which hurts the flow of nutrients downstream, ruins the natural riverscape, and can cause the area behind the dam to become more shallow.

Dams carry numerous drawbacks, but one benefit of dams that they do not increase air pollution. Hydroelectric power is considered to be one of the cleanest forms of energy.

The energy output from a hydro-electric dam is substantial enough to result in most of America's major rivers being damned. The more pressing issues that these damns cause are changes to the aquatic ecosystem and restriction of movement for migratory fish species. 

While there are federal wetland conservation laws geared at protecting aquatic habitat, there is currently no federal ban on constructing hydroelectric dams, and the returned energy investment for hydroelectric is quite substantial. The dominant issue with expanding hydroelectric energy production is that in the U.S., we have dammed most if not all of the significant energy-producing water bodies. Further expansion of hydroelectric power will not yield as much energy as the current dams operating in the U.S.

Wattage is a measurement of electricity being generated over time. When compared to the 5,000 watt output of a conventional generator, a single solar panel generating watts cannot generate the same rate of electrical output. As a result, we will need multiple panels to generate an equal amount of electrical output. We can use dimensional analysis to determine how many panels we would need:

This calculation tells us that Charlie will need 25 standard solar panels to generate the same amount of electricity as one gasoline generator.

The correct response is photovoltaic cells. These photovoltaic cells are the reason we can capture and use sunlight to power our homes and cars. Technological advances in photovoltaic cells have made solar energy economically viable over the past decade. Eukaryotic cells are a type of living organisms, while thermal and radiation cells are made-up terms.

Ethanol can be made from cellulosic—or woody—plants such as switchgrass, poplars, and willows. Cellulose, or plant tissue, sequesters a huge amount of carbon and contains a great deal more energy per unit than corn. Scientists believe that within the next two decades, research will have figured out how to quickly and efficiently break down plant tissue for fermentation into fuel.

Biodiesel made from corn is environmentally safe to burn; however, it requires a great deal of water and resources to grow and harvest.

Agricultural wastes can have a negative impact on air quality due to the pollutants they generate when burned, such as sulfur oxide, nitrogen oxide, and carbon monoxide.