Which gases are used for diagnostics and are not considered hazardous pollutants?

The choice of carbon dioxide (CO2) and oxygen (O2) as the correct answer is based on their common use in diagnostics and their classification with respect to hazardous pollutants. CO2 and O2 are often utilized in various diagnostic processes, especially in environmental monitoring and medical applications.

Carbon dioxide is a naturally occurring gas that plays a key role in respiratory processes and is not considered a hazardous air pollutant at typical ambient levels. Oxygen is essential for life and is non-toxic in the concentrations typically encountered in air. Neither of these gases is associated with significant health risks when encountered in normal atmospheric concentrations, making them suitable for diagnostic use without the concerns that accompany hazardous pollutants.

In contrast, the other options include gases that have recognized health, environmental, or safety implications and are often regulated due to their potential harmful effects. For example, sulfur dioxide (SO2) and nitrogen dioxide (NO2) are known for their harmful respiratory effects and are classified as hazardous pollutants. Similarly, methane (CH4) and ethylene (C2H4) can pose environmental risks and fall under various regulatory frameworks. Thus, the distinction made by the correct choice emphasizes the safety and diagnostic suitability of CO2 and O2 compared to the other gases listed.