单项选择题
A.forest fires B.smoke from industrial areas C.dust or sand storms D.water droplets with the relative humidity more than 95%
A.low barometric pressure B.high barometric pressure C.falling barometric pressure D.pumping barometric pressure
A.Is 50% water saturated B.Is 90% water saturated C.Temperature is greater than the dew point temperature D.Temperature is equal to，or below the dew point temperature