The air quality index measures

a. the level of carbon monoxide over the previous ten days.
b. deterioration in the ozone layer.
c. concentration of five air pollutants of concern to health.
d. amount that visibility is diminished by smog.

C. concentration of five air pollutants of concern to health.

Explanation:

The Air Quality Index (AQI) is a measurement of air quality calculated from five major air pollutants, particulate matter, carbon monoxide, nitrogen monoxide, sulfur dioxide and ground-level ozone. This index is based on national air quality standards and it focuses on health effects when breathing polluted air, the higher the AQI value, the greater the pollution and health concern.

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