(S (S (NP (BASENP (NNP United) (NNPS Airlines)) (CC and) (BASENP (NNP Virgin) (NNP America))) (VP (VP (VP (VBD offered) (BASENP (NNS passengers))) (NP (BASENP (DT the) (NN opportunity)) (SBAR (S (VP (TO to) (VP (VP (VB change) (BASENP (DT the) (NNS flights))) (PP (IN at) (BASENP (DT no) (JJ extra) (NN cost))))))))) (PP (IN until) (BASENP (NNP Monday))))) (PERIOD .))