39. Industrialized countries have worked to ____ levels of sulfur dioxide, smog, and smoke in order to improve people’s health. (A) duplicate (B) increase (C) reduce (D) demarcate

40. Scientists say feral pigeons are a valuable way of probing contaminants in environment, ___ they are exposed to the same air, water, food and other factors as humans, and don’t venture far from home. (A) although (B) hence (C) because of (D) since