may be the performances of their careers 
(ROOT (S (VP (MD may) (VP (VB be) (NP (NP (DT the) (NNS performances)) (PP (IN of) (NP (PRP$ their) (NNS careers))))))))
(ROOT (S (VP (MD ) (VP (VB ) (NP (NP (DT ) (NNS )) (PP (IN ) (NP (PRP$ ) (NNS ))))))))