Syntactically,TrP,which is a case-marking head phrase with a transitivity feature and EPP,exists between vP and VP.The presence of Tr is able to derive not only wh-questions in which superiority effects arise which MLC can explain,but also questions in which superiority effects disappear which the equidistance and AgrOP models cannot explain.Semantically,English non-D-linked wh-questions' inadequacy of SP readings,D-linked wh-words' specificity and wh-arguments' choice functions all constrain the appearance of superiority effects and their acceptability.