(S (S (S (NP (BASENP (NNS Tanks)) (PP (IN of) (NP (NP (BASENP (NN oxygen)) (COMMA ,) (BASENP (NN helium))) (CC and) (BASENP (NN acetylene))))) (VP (VBD began) (S (VP (TO to) (VP (VB explode)))))) (SBAR (IN after) (S (NP (BASENP (DT a) (NN connector)) (VP (VBN used) (S (VP (TO to) (VP (VP (VB join) (BASENP (NN acetylene) (NNS tanks))) (PP (IN during) (BASENP (DT the) (NN filling) (NN process)))))))) (VP (VBD malfunctioned))))) (PERIOD .))