Westwood, John
Gramedia, 2006
Buku Teks
Berry, Ralph
London : ROUTLEDGE, 1994,
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Berry, Ralph
Routledge, 1994
Buku Teks SO
Purves, Alan C. (Alan Carroll), 1931-1996
Harcourt Brace Jovanovich, 1984
Buku Teks SO
Murray, Rowena
Open University Press, 2002
Buku Teks