Why is solar power considered a renewable resource? a) solar power does not cost anything. b) the sun shines all the time everywhere. c) the solar winds increase during the ages. d) the supply of solar power will not run out.

Solar power is a renewable resource because D) solar power supply will not run out.

Renewable energy describes the energy that cannot be depleted over time since the source is forever available. The solar power provided by sun rays is an example of solar energy since it is naturally occurring. Other benefits of solar energy include environmental cleanliness due to non-emittance of green-house gases.  

It is also cost-effective and efficient. Other examples of renewable resources include wind, hydro, geothermal, and biomass. The opposite on renewable resources is non-renewable resources in terms of fossils fuels such as coal, oil, nuclear, etc.

81 is a composite number.