Why Do Air Pollution Dust Domes Often Form in Urban

Why do air pollution dust domes often form in urban areas?

A) Urban areas are usually in valleys.
B) There are a high number of mobile sources that emit air pollutants.
C) Pavement and rooftops concentrate heat and create relatively stable air masses that lift and keep pollutants in the air.
D) Large cities are usually located on peninsulas or islands.