( PAHs ( ( are ( ( a group ) ( of ( ( ( ( ( ( chemicals ( that ( are ( formed ( during ( ( the ( incomplete burning ) ) ( of ( coal ( , ( ( oil and ) gas ) ) ) ) ) ) ) ) ) ) , ) garbage ) , ) or ) ( ( other ( organic substances ) ) ( -LRB- ( 12 -RRB- ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS PAHs)) (VP (VBP are) (NP (NP (DT a) (NN group)) (PP (IN of) (NP (NP (NP (NNS chemicals)) (SBAR (WHNP (WDT that)) (S (VP (VBP are) (VP (VBN formed) (PP (IN during) (NP (NP (DT the) (JJ incomplete) (NN burning)) (PP (IN of) (NP (NN coal) (, ,) (NN oil) (CC and) (NN gas)))))))))) (, ,) (NP (NN garbage)) (, ,) (CC or) (NP (NP (JJ other) (JJ organic) (NNS substances)) (PRN (-LRB- -LRB-) (NP (CD 12)) (-RRB- -RRB-))))))) (. .)))
PAHs are a group of chemicals that are formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances ( 12 ).
(ROOT (S (NP (NP (DT The) (NN action)) (PP (IN of) (S (VP (VBG burning) (NP (NP (DT a) (NN piece)) (PP (IN of) (NP (NN wood)))))))) (VP (VBZ forms) (NP (DT a) (JJ different) (NN chemical) (NN substance))) (. .)))