1. Wind power is one of the earliest ................. sources of energy used by humankind.

A. alternative B. polluted C. harmful D. exhausted
2. Nobody can predict exactly when supplies of fossil fuels will be ................. However, we all know they quickly reduce in quantity.

A. installed B. polluted C. harmful D. exhausted
3.Biogas can be used to ................. natural gas in cooking, heating, or electrical generation.

A. use up B. replace C. pollute D. install
4. Solar energy can be used for ................. electricity, and for hot water heating.

A. investing B. polluting C. generating D. installing