Throughout history man has changed his physical environment in order to (36) his way of life. With the tools of technology he has (37) many physical features of the earth. He has (38) woodlands into farmland, and made lakes and reservoirs out of rivers for irrigation (39) or hydroelectric power. Man has also modified the face of the earth by draining marshes and cutting’ through mountains to build roads and railways.
However, man’s changes to the physical environment have not always had (40) results. Today, pollution of the air and water is an (41) danger to the health of the planet. Each day thousands of tons of gases come out of the exhausts of motor (42) , smoke from factories pollutes the air of the industrialized areas and the (43) areas of countryside. The air in cities is becoming increasingly unhealthy.
(44) In the sea, pollution from oil is increasing and is killing enormous numbers of algae, fish and birds. The whole ecological balance of the sea is being changed. The same problem exists in rivers. (45) . (46) .