We use many different energy sources to do work for us. Energy sources are classified into two groups - renewable and nonrenewable. Non-renewable energy sources are those that take millions of years to form and will run out some day. It is energy that comes from fossil fuels such as coal, crude oil, and natural gas. Fossil fuels are mainly made up of carbon and were formed millions of years ago. The chemical reaction which takes place when we burn fossil fuels releases carbon compounds such as carbon monoxide and carbon dioxide into the air. Carbon dioxide is one of the green house gases which is contributing to global warming and air pollution. On the other hand, renewable energy sources will never run out, are better for the environment and do not cause pollution. Renewable energy is often called green energy because it is a natural energy, always available and does not have to be formed like nonrenewable energy. The green energy is always there. For example, the Sun consistently shines, water is abundant, and the winds blow throughout the year. The five types include solar, energy from the Sun; geothermal, energy from heat within the Earth; hydroelectric, energy from moving water; biomass, energy from dead plants and microorganisms and finally, energy from the wind.

Thì hiện tại hoàn thành chỉ:

A. Kinh nghiệm 

B. Một hành động, sự việc đã bắt đầu từ trong quá khứ, kéo dài đến hiện tại và có thể tiếp tục tiếp diễn trong tương lai.

C. Cả hai ý trên