3Krashen S. D. , C. Jones,S. Zelinski and C. Usprich. How Important Is Instruction[J]. ELT Journal Vol. 32:257 -261,1978.
4Caroll B.J. Foreign Language Proficiency Levels Attained by Language Majors Near Graduation from College [J].Foreign Language Annals, vol. 1:134 - 151,1967.