(S (S (BASENP (NN One) (JJ possible) (NN cause)) (VP (VBZ is) (SBAR (IN that) (S (BASENP (DT the) (NN driver)) (VP (VP (VBD fell) (ADVP (RB asleep))) (PP (IN at) (BASENP (DT the) (NN wheel)))))))) (PERIOD .))