An atom is to a molecule as A. a house cat is to a lion.
B. a sentence is to a word.
C. a brick is to a house.
D. a nail is to a hammer

Air pollution: “combustion results in gases such as nitrogen oxides, nitrogen dioxide and particulates. the main causes of air pollution include transport, domestic combustion and industrial processes. pollutants from these sources can cause problems in the immediate vicinity and a distance away.”