2[2]Cenoz,J.& Hufeisen,B.& Jessner,U.Cross-linguistic Influence in Third Language Acquisition:Psychological Perspective[M].Cleve don:MultilingualMatters LTD,2001.
3[3]Cook,V.Evidence for multi-competence[J].Language Learning,1992,(4):557-591.
4[4]Hoffmann,C.Towards a description of trilingual competence[J].International Journal of Bilingualism,2001,(1):1-17.