Which of the following air pollutants has shown the greatest decline in recent years?

Options

A. lead
B. nitrogen oxides
C. carbon monoxide
D. sulfur dioxide

Answer

To determine which air pollutant has shown the greatest decline in recent years, we can follow these steps:

### Step 1: Identify the Air Pollutants
First, we need to list the air pollutants mentioned in the question. The common air pollutants include:
- Lead
- Nitrogen Oxides (NOx)
- Carbon Monoxide (CO)
- Sulfur Dioxide (SO2)

### Step 2: Analyze the Sources of Each Pollutant
Next, we should consider the sources of these pollutants:
- **Lead**: Historically used in aviation fuels and other industrial processes.
- **Nitrogen Oxides**: Primarily emitted from vehicles and industrial processes.
- **Carbon Monoxide**: Mainly produced from incomplete combustion of fossil fuels.
- **Sulfur Dioxide**: Emitted from burning fossil fuels, especially coal and oil.

### Step 3: Evaluate Recent Trends in Pollution Levels
Now, we need to look at recent trends regarding the emissions of these pollutants:
- **Lead**: There has been a significant reduction in lead emissions due to regulations banning leaded gasoline and reducing lead in industrial processes.
- **Nitrogen Oxides, Carbon Monoxide, and Sulfur Dioxide**: While there have been efforts to reduce these pollutants, their levels have not declined as dramatically as lead in recent years.

### Step 4: Conclusion
Based on the analysis, lead has shown the greatest decline in recent years due to strict regulations and reduced usage in aviation and industry. Therefore, the correct answer is **Lead**.