Emissions from heavy-duty diesel trucks and buses in recent years have:

• A. Been significantly reduced.
• B. Increased significantly.
• C. Remained constant.
• D. Fluctuated without a clear trend.

Answer: (B)

Modern heavy-duty diesel engines have become much cleaner thanks to tighter emission standards and advanced aftertreatment. Technologies like diesel particulate filters reduce soot, while selective catalytic reduction cuts NOx, all aided by low-sulfur fuel and newer engine designs. As a result, new trucks and buses emit far less PM and NOx than older models, and fleets are gradually replacing the higher-emitting ones. While real-world factors like maintenance, operation patterns, and idling can cause some short-term variations, the overall trend in recent years is a significant reduction in emissions.