例三：man and his environment(人和环境)
man and his environment
throughout history man has changed his physical environment in order to improve his way of life. with the tools of technology he has altered many physical features of the earth. he has transformed woodlands and prairies into farms and made lakes and reserviors out of rivers for irrigation purposes 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 beneficial results. today, pollution of the air and water is an increasing danger to the health of the planet. each day thousands of tons of gases come out of the exhausts of motor vehicles. smoke from factories pollutes the air of industrialized areas and the surrounding areas of the co