Read the passage and complete the sentences

Energy is fundament to human beings. Many poor people in developing countries do not have modern sources of energy like electricity or natural gas, with which their life can be improved.

People who live in mountainous areas have to gather wood for fuel. This takes a lot of time. For many people living in rural areas, biogas is the largest energy resource available. The main use of biogas is for cooking and heating, but it can also provide energy for public transport. As biogas is smoke-free, it helps the problem of indoor air pollution. Moreover, it is made from plant waste and animal manure. They cost almost nothing.

The tendency to use renewable energy sources in developing sources in developing countries is on the increase as non-renewable ones are running out. In the future, the wind and the sun will be uses as the most important environmentally friendly energy sources.

Question: In the future, the ________________________ will be used as the main environmentally friendly energy sources.

is a single-wheel bike