<prompt> The best indicator for determining the general level of air pollution in an urban area isSulphur dioxideCarbon dioxideHydrogenNitrogen  <response>: A Sulphur dioxide Best indicators of air pollution are sulphur dioxide, smoke, suspended particles. Monitoring of air pollution Best indicators of air pollution are sulphur dioxide, smoke and suspended particles These are monitored on a daily basis over several sites. The results are then collected by a central agency. Sulphur dioxide: This gas is a major contaminant in many urban and industrial areas. Its concentration is estimated in all air pollution surveys, 