In a windowing environment in which more than one program or document can be available to the user, open but currently inactive (2) _______ are said to be in the background.

There are many sources of pollution in our modern world. At present, the most severe sources are acid rain, car exhaust fumes and oil spills. Factory chimneys give (1)_________ smoke that contains sulfur dioxide and nitrogen oxide. These gases combine with the moisture in the atmosphere to (2)_________ sulfuric acid and nitric acid. When it rains, these acids dissolve in the rain and turn it acidic. Acid rain is thought to be the worst pollution problem of all. It has (3)_________ aquatic life by turning many lake into lifeless bodies of water. In these areas, there is no limestone in the rocks to neutralize the acid rain naturally. One remedy is to pump limestone into the acidic lakes. (4)_________, it is not possible to do this on a large enough scale to save all the lakes. Car exhaust fumes contain carbon monoxide and lead which are highly poisonous. In big cities, the exhaust fumes build (5)_________ and pose a health hazard to human beings. Pollution of the sea by oil threatens marine life. Much of the oil comes from ships that clean their fuel tanks while at sea. Offshore oil wells also discharge vast amounts of oil into the sea. Pollution of the sea can also be caused by oil spills. An oil spill may occur as a result of accidents involving oil tankers