2Burton H R.Relationship between tobacco-specific nitrosamines and nitrite from different air-cured tobacco varieties[ J].J.Agric.Food Chem.,1994,42(9):2007-2011.
6Orozco M V.Qualitative and quantitative determination of alkaloids in tobacco residues by titrimetry and ultraviolet spectroscopy [J].Tob.Abs.,1975,(6):802.
2[1]Kumaradivelu B. Language-learning tasks: teacher intention and learner interpretation[J]. ELT Journal 1991,45(2):98-107.
3[2]Nunan D. Designing Tasks for the Communicative Classroom[M]. Cambridge: Cambridge University Press. 1989.
4[3]Pica T, Kanagy R, and Falodun J. Choosing and using communicative tasks for second language instruction[A]. G Crookes and S M Gass (eds.) .Tasks and Language Learning: Integrating Theory and Practice[Z]. Clevedon, Avon: Multilinggual Matters.1993.
5[4]Richards J C and Rodgers T S.Approaches and Methods in Language Teaching[M]. Cambridge: Cambridge University Press.1986.
6[5]Skehan P. A Cognitive Approach to Language Learning[M]. Oxford: Oxford University Press.1998.
7[6]Widdowson H G. Aspects of Language Teaching[M]. Oxford: Oxford University Press.1990.
8[7]Williams M, Bueden R L. Psychology for Language Teachers[M]. Cambridge: Cambridge University Press.1997.
9Davis D L. Waxes and lipids in leaf and their relation to smoking quality and aroma. Rec. Adv. Tob. Sci. ,1976,80～109.
10Enzell C R, Wahlberg I. Leaf composition in relation to smoking quality and aroma. Rec. Adv. Tob. Sci. ,1980,64～121.
