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.