Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions.

One of the most discussed problems is pollution of our environment. Factories, cars, power stations, and fertilizers have emitted a large amount of smoke and pollutants into the air. Consequently, water, forests, and the air we breathe are polluted.  Only 50 % of people drink drinkable water.

Some lakes have dried up and our climate has changed.  Lately, the ozone layer is becoming more damaged and as a result dangerous illness have increased.  Cancer is the most popular illness we can see.

Another problem, which is quite serious, is considered the Green House Effect, which is made up by carbon dioxide and methane. The temperature has increased about 0.6 degrees Celsius since the beginning of 20th century. As the result of global warming the polar ice-caps melt. The more the icecaps melt the quicker the sea level rises and millions of people can die or lose their homes.

Forest destruction seems to be a serious problem. In tropical forests there are rare species of animals, flowers, trees and people have destroyed them. As a result, animals are without shelters, special kinds of flowers do not decorate our surroundings and the soil is iiot stabilized.  Everybody must think about these problems because we do not know when nature shows us how strong it can be. A recent example: Tsunami in Asia. Thousands of people died.  That is a strong enough warning.

Which is not referred to as one of the causes of pollution?