The EPA sets national air-quality standards for common air pollutants. The graph shows the change in the concentrations of these pollutants over time.
Which conclusion do the data in the graph support?

A Concentrations of all pollutants have decreased.
B. Monitoring concentrations of pollutants is too expensive.
O C. Concentrations of all pollutants have increased.
O D. Monitoring concentrations of pollutants has been ineffective.