Directions: Identify the topic sentence in the following paragraph. The forest is a sanitary agent. It is constantly eliminating impurities from the earth and the air. Trees check, sweep, and filter from the air quantities of filthy, germ-laden dust. Their leaves absorb poisonous gases from the air. Roots assist in drainage, and absorb impurities from the soil. Roots give off acids, and these acids, together with the acids released by the fallen, decaying leaves, have a sterilizing effect upon the soil. Trees help to keep the earth sweet and clean, and water which comes from a forested watershed is likely to b e pure. Many unsanitary areas have been redeemed and rendered healthy by tree planting.