Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions from 29 to 35.

Energy is fundamental to human beings, especially to poor people - the one- third of humanity that does not have modern energy supplies like electricity or natural gas, with which their life could be improved.

Many people who live in countryside and mountainous areas have to gather fuel wood. This takes a lot of time. Moreover, it causes indoor air pollution because of heavy smoke. For many people living in rural areas in developing countries of the world, biogas is the large energy resources available and costs almost nothing. 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 solve the problem of indoor air pollution. Moreover, plant waste and animal manure cost almost nothing.

The tendency to use renewable energy sources in developing countries is on the increase when non-renewable ones are running out. In the near future, people in the developing countries may use wind or sea waves as the environment-friendly energy sources.

What is the role of energy to human beings?