( ( People ( cause pollution ) ) ( , ( people ( ( can ( stop it ) ) . ) ) ) )
(ROOT (S (S (NP (NNS People)) (VP (VBP cause) (NP (NN pollution)))) (, ,) (NP (NNS people)) (VP (MD can) (VP (VB stop) (NP (PRP it)))) (. .)))
People cause pollution, people can stop it.
(ROOT (S (NP (NNS People)) (VP (VBD stopped) (S (VP (VBG adding) (NP (NN lead)) (PP (TO to) (NP (NP (NN gasoline)) (PP (RB because) (IN of) (NP (JJ environmental) (NN pollution)))))))) (. .)))