( Automobiles ( ( are ( ( the ( primary source ) ) ( of ( air pollution ) ) ) ) . ) )
(ROOT (S (NP (NNS Automobiles)) (VP (VBP are) (NP (NP (DT the) (JJ primary) (NN source)) (PP (IN of) (NP (NN air) (NN pollution))))) (. .)))
Automobiles are the primary source of air pollution.
(ROOT (S (NP (JJ Primary) (NNS pollutants)) (VP (VBP enter) (NP (DT the) (NN air)) (ADVP (RB directly))) (. .)))