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ASTD Press, 2008
Buku Teks SO
Syaiful Iman
2018
UI - Skripsi Membership
Pittman, G. A.
Longmans, 1967
Buku Teks SO
Pittman, Al
Breakwater Books Limited, 1978
Buku Teks SO
edited by Andrew Gonzalez and David Thomas
Summer Institute of Linguistics (Philippines) : Linguistic Society of the Philippines, 1981
Buku Teks SO