Final answer:
Lead levels fell by 94% due to the elimination of leaded gasoline.
Step-by-step explanation:
The correct answer is b. elimination of leaded gasoline. Lead levels fell by 94% due to the elimination of leaded gasoline. Lead was commonly used as an additive in gasoline to boost engine performance. However, it was discovered that leaded gasoline contributed significantly to lead pollution in the environment, which posed serious health risks, especially to children. As a result, leaded gasoline was phased out and replaced with unleaded gasoline, leading to a significant decrease in lead levels.