Which two types of biomass are commonly used as energy resources?

Biomass refers to organic materials that can be used as a source of energy. Wood and food waste are two of the most common types of biomass utilized for energy production. Wood is frequently used for heating, and it can also be converted into biofuels such as ethanol or biodiesel. Food waste, on the other hand, can be processed through various methods like anaerobic digestion to produce biogas, which can serve as a renewable energy source.

In contrast, the other options listed involve materials that are not classified as biomass. Plastic and metal are synthetic or inorganic materials, and although they can sometimes be recycled for energy, they do not fall within the conventional definition of biomass. Coal, petroleum, and natural gas are fossil fuels derived from ancient organic matter but are processed forms of energy that typically do not constitute biomass in current usage. Thus, wood and food are the correct choices as they specifically represent natural, renewable resources that can be regenerated and are commonly harnessed for energy purposes.