Natural gas is a secondary source of energy derived from the decomposition of organic matter over millions of years.

According to the law of diminishing returns, as more of a variable input is added, the marginal product of that input will eventually decrease due to diminishing marginal productivity.

Physical market is not a type of energy market. Spot market, forward market, and futures market are all types of energy markets.

Energy efficiency is the ability to use less energy to perform the same task or produce the same amount of output.

Fossil fuels, such as coal, oil, and natural gas, are non-renewable energy sources.

Energy independence refers to a country's ability to meet its energy needs without relying on imports from other countries.

Carbon pricing is a policy mechanism that puts a price on carbon emissions, making it more expensive to pollute. It is not a type of energy subsidy.

Energy poverty refers to the lack of access to reliable and affordable energy services, which can have significant social, economic, and environmental consequences.

Peak oil refers to the point at which global oil production reaches its maximum and begins to decline due to factors such as resource depletion and technological limitations.

Fossil fuel power plants are not a type of energy storage technology. They are used to generate electricity from fossil fuels.

Negative externalities in energy production refer to the environmental, health, economic, and social costs that are imposed on society as a result of energy production activities.

Energy subsidies are not a type of energy policy instrument. They are financial incentives provided to promote the production or consumption of certain energy sources.

Energy transition refers to the shift from fossil fuels to renewable energy sources and other sustainable energy technologies.

Carbon offset is not a type of renewable energy certificate. It is a financial instrument that represents a reduction in greenhouse gas emissions.

Energy justice encompasses fair and equitable access to energy resources and services, distribution of energy benefits and burdens, representation of energy stakeholders in decision-making, and pricing of energy products and services.