Read the followingpassage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.

One of the most urgent environment problems in the world today is the shortage of clean water.Having clean drinking water is a basic human right.But acid rain, industrial pollution and garbage have made many sources of water undrinkable.Lakes and even entire seas have become large pools of poison.LakeBaikalinRussiais one of largest lake in the world.It contains a rich variety of animals and plants, including 1, 300 rare species that do not exist anywhere else in the world.But they are being destroyed by the massive volumes of industrial pollutants which pour into the lake everyday.Even where law existed, the government did not have the power to enforce them.Most industries simply ignore the regulations.TheMediterranean Seaoccupies 1% of the world's water surface.But it is the dumping ground for 50% of all marine pollution.Almost sixteen countries regularly throw industrial wastes a few miles off shore.Water is free to everyone.A few years ago people thought that the supply of clean water in the world was limitless.Today, many water supplies have been ruined by pollution and sewage.Clean water is now scarce, and we are at last beginning to respect this precious source.We should do something now.

School children, office-workers, and others fill (23)______ roads, bus-stops and the vehicles.