Which of these energy resources does not pollute when used

Solar power: solar power converts the sun's energy into energy we can use.Nuclear power: heat is used to generate steam that drives a steam turbine connected to a generator that produces electricity.Hydroelectric energy: hydroelectric power plants use a dam on a river to store water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity.Wave energy: wave energy proves itself to be a valuable contributor to alternative energy fuel sources with wave energy converters.Biofuels: biofuels make use of animal and plant life to create energy.Natural gas: emits greenhouse gases, but the amount is lower than with fossil fuels.Geothermal power: geothermal power is about extracting energy from the ground Wind energy: it uses wind as energy and it makes use of a renewable source and generates no waste.Biomass energy: biomass energy comes in a number of forms. For example, burning wood has been used for thousands of years to create heat, but more recent advancements have also seen waste, such as that in landfills, and alcohol products used for similar purposes.Tidal energy: tidal energy uses the power of water to generate energyHydrogen gas: hydrogen is a completely clean burning fuel. Once produced, hydrogen gas cells emit only water vapor and warm air.

There are no metaphors.