Read the following text and answer the questions no. 3 and 4:

Humans can neither change the sun's radiation for the earth's orbit around the sun. But they can control the increase in the amount of greenhouse gases and its effect on the atmosphere. Only during the last hundred years the carbon dioxide concentration has been raised alarmingly in the atmosphere and we humans can be held responsible for this.

The main cause of the increase in carbon dioxide level in the atmosphere is the burning of fossil fuels. Since the end of the 19 century. Industrial activities increased rapidly giving rise to many factories. These factories required energy, which was produced through the combustion of coal. Besides coal, other sources of energy such as mineral oil and natural gas were also burned to heat our houses, run cars and airplanes or to produce electricity. Nowadays, about $5 million barrels of crude oil are burned daily. Every time a fossil raw material is burned, it releases carbon dioxide into the air.

Therefore, it is clear that more and more greenhouse gases like carbon dioxide are being generated worldwide by humans. Moreover, we are also strengthening the greenhouse effect by deforestation, which means cutting down trees. Every year enormous areas of forests are destroyed by people to obtain wood and to clear regions for mining and to create pasture. This loss of the forest causes dual problems. Trees that are burned up release large volumes of carbon dioxide gas into the air. On the other hand, as forests absorb a lot of carbon dioxide from the air and deliver oxygen instead, we also destroy an important storehouse of carbon dioxide when we clear forests.