Ewell, Peter T.
Ithaca, N.Y. Department of Agricultural Economics Cornell University 1984,
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Martin, Anne
Routledger Kegan Paul , 1958
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Hubbard, Leonard E., author
MacMillan, 1939
Buku Teks
Wilcox, Walter W., author
Prentice-Hall, 1951
Buku Teks
Ross, Robert C., author
McGraw-Hill, 1951
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