Weeks, Kent R.
William Morrow, 1997
Buku Teks
Michael Kalvius, Paul Kienle
Springer, 2012
eBooks
Hutchison, Bruce, author
Longmans Green, 1953
Buku Teks
Donaldson, Norman
Bowling Green University Popular Press, 1971
Buku Teks
Winston, Robert
BBC Worldwide Limited, 2004
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