سؤال قراءة ستيب محلول — الفكرة الرئيسية (تدريب 5390)

(الوقود الحفري) (1) Fossil fuels include coal, oil and natural gas, formed from the remains of plants and animals which lived millions of years ago and obtained energy originally from the Sun. At present they are our main energy source. Predictions vary as to how long they will last since this depends on what reserves are recoverable and on the future demands of a world population expected to increase from about 7,000 million in 2011 to at least 7,600 million by the year 2050. Some estimates say oil and gas will run low early in the present century but coal should last for 200 years or so. (2) Burning fossil fuels in power stations and in cars pollutes the atmosphere with harmful gases such as carbon dioxide and sulfur dioxide. Carbon dioxide emission aggravates the greenhouse effect and increases global warming. It is not immediately feasible to prevent large amounts of carbon dioxide entering the atmosphere, but less is produced by burning natural gas than by burning oil or coal; burning coal produces most carbon dioxide for each unit of energy produced. (3) When coal and oil are but they also produce sulfur dioxide which causes acid rain. The sulfur dioxide can be extracted from the waste gases so it does not enter the atmosphere or the sulfur can be removed from the fuel before combustion, but these are both costly and risky processes which increase the price of electricity produced using these measures. What is the main topic of the passage?