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American Enterprise Institute for Public Policy Research, 1977
Buku Teks SO
SR Indonesia,
Majalah, Jurnal, Buletin
Gardner, Richard N.
Dell Publishing, 1965
Buku Teks SO
edited by Gordon Greenwood and Norman Harper
F.W. Cheshire, 1957
Buku Teks SO
Lovering, T.S.
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Buku Teks