( 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 (NP (NNP Ash)) (SBAR (WHNP (WDT that)) (S (VP (VBZ enters) (NP (DT the) (NN air)) (PP (ADVP (RB naturally)) (IN as) (NP (NP (DT a) (NN result)) (PP (IN of) (NP (DT a) (NN volcano) (NN eruption))))))))) (VP (VBZ is) (VP (VBN classified) (PP (IN as) (NP (DT a) (JJ primary) (NN pollutant))))) (. .)))