( It ( ( is ( ( only ( the ( carbon-14 isotope ) ) ) ( that ( is radioactive ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (NP (NP (RB only) (DT the) (JJ carbon-14) (NN isotope)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (ADJP (JJ radioactive))))))) (. .)))
It is only the carbon-14 isotope that is radioactive.
(ROOT (S (NP (NP (NNP Carbon) (CD 14) (NN isotope)) (PP (IN of) (NP (NN carbon)))) (VP (VBZ is) (ADVP (RB typically)) (VP (VBN used) (PP (TO to) (NP (NN date) (JJ ancient) (NNS items))))) (. .)))