What is an important factor that could make a renewable energy source a realistic alternative to nonrenewable energy sources? A. Stop the use of low-carbon, low-energy yield brown coal in power plants, and switch over to more efficient high-carbon, high-energy yield hard coal.

B. Continue to mine coal at current rates, but use the coal to create new types of photovoltaic cells for use in solar panels on large solar farms.

C. Decrease the cost of building wind turbines to decrease the initial cost of wind energy so that it is closer to the cost of energy currently produced from burning coal.

D. Increase the volume of natural gas extracted from wells through hydraulic fracturing to decrease the amount of coal that has to be mined for combustion.

C.  

Explanation:

Renewable energy sources (e.g., solar energy, wind energy, hydrothermal energy) represent the most important alternative to reduce global warming effects due to the use of fossil fuels. However, the use of renewable energy sources is nowadays more expensive than those derived from fossil fuels. For example, it has been calculated that solar energy costs around 20 cents more per KWH than coal or gas. Wind energy is even more expensive than solar energy. In consequence, it is expected that in the near future renewable energy production costs decrease in order to make this type of energy source cheaper than fossil fuels.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.

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