Many human activities over the last 200 years have been responsible (30) ________ polluting the air and damaging people’s health. Automobile engines and power plants burn combustible (31) ________ like gasoline and coal and allow toxic gases and smoke to escape into the air. Some pollutants destroy the ozone layer, which is the thin blanket of gases that (32) ________ Earth from the Sun’s dangerous ultraviolet rays. Other pollutants contribute to (33) ________ warming by adding to the planet’s natural greenhouse effect. Still others create acid rain, a phenomenon that has disastrous (34) ________ on lake and forest habitats. Air pollution is not limited to industrial areas. Depending on the direction of the wind and its force, air pollutants may (35) ________ spread to other countries very far from the source of the pollution. Big cities like Los Angeles and Mexico City are*often covered by smog, a fog that occurs (36) ________ air pollution. In 1952, the City of London, England, was enveloped by smog so thick that people on the streets had to feel their way around by (37) ________ the walls of buildings! 30. A. for B. of C. to D. with 31. A. chemicals B. means C. fuels D. matters 32. A. covers B. protects C. stops D. damages 33. A. environmental B. thermal C. temperature D. global 34. A. affects B. causes C. sources D. effects 35. A. never B. even C. ever D. hardly 36. A. because of B. in spite of C. instead of D. due of 37. A. climbing B. walking C. touching D. putting up