2Krashen, S. 1988. Second Language Acquisition [M]. London: Pergamon Press.
3Richards, J. C. & Rodgers, T. 2001. Approaches and Methods in Language Teaching [M]. Cambridge: Cambridge University Press.
4Swain, M. 1985. Communicative competence: Some roles of comprehensible input and comprehensible output in development [A]. In S. Gass & C. Madden (Eds.). Input in Second LanguageAcquisition [C]. New York: Newbury House, 235-256.
