Pollution comes from modern society. Motor vehicles are the fastest growing source of air pollution today, pumping out a toxic mixture of health-threatening (1) ______. Traffic fumes are largely (2) ______ for the increasing number of smogs which pollute out towns, cities and rural areas. In fact, cars produce 15 percent of worldwide greenhouse gases. It is estimated that the number of cars on the world’s road will rise to 1.5 billion in the next 20 years. Besides, a lot of pollutants are (3) ______ into the atmosphere by industrial processes. For example, when coal is burnt in power stations to generate electricity, Sulphur dioxide and nitrous oxides are produced. These are the two major gases which (4) ______ acid rain. Another source of pollution is from waste disposal. When rubbish is burned, the smoke and gases produced often contain pollutants such as dioxins, which are a (5) ______ to human health and the environment.