Cameron, Silver Donald, author
McClelland and Stewar, 1978
Buku Teks
Edwards, Edward, author
Routledge/Thoemmes Press, 1996
Buku Referensi
UK : The Association of Commonwealth Universities, 1988,
Buku Referensi
Corrigan, Timothy
Scott, Foresman and Company, 1989
Buku Teks
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