A、It keeps track of how clean or populated the air is. B、It can predict the temperature of the next days. C、It can direct people

The one thing you need to know is that if you were like many who suffer when the air quality is poor, relief could be just a click away. There’s something called the Air Quality Index, or (3)the AQI, and it keeps track of how clean or populated the air is on a day to day basis. The Environmental Protection Agency calculates the AQI on five major pollutants that are regulated by the Clean Air Act.
Ground level ozone and particle pollution are the greatest threat to human health. The Air Quality Index runs from zero to 500, with 500 being the most unhealthy. (4)Over 100 could be considered unhealthy for sensitive groups. We’re talking about people with asthma as well as heart conditions. Everyone will start feeling the effects of poor air quality when the number reaches 150 or above. Anything over 300 is considered hazardous air quality. It’s encouraged on these high level days to limit your outdoor activities and stay indoors. Exercising outdoors during one of these high level days could result in serious health consequences. So, before you go outside, check the AQI to protect yourself and to have better health.
Questions 3 and 4 are based on the news report you have just heard.
3.What can we know about AQI?
4.When is it unhealthy for sensitive people?