Concept:The correct word must describe a gradual decline in the quality or condition of the environment.Explanation:Pollution refers to adding harmful substances to the environment.Damage is too general and does not clearly express a steady decline.Destruction means complete ruin, which is too extreme.Degradation means a gradual fall in quality or condition.Therefore, the phrase "degradation of the environment" is the most accurate and natural completion.Answer:C. degradation