Eckersley, C. E. (Charles Ewart), 1892-1967, author
Longmans, Green and Co., 1953
Buku Referensi
Fucilla, Joseph G., author
Follett Publishing Company, 1964
Buku Referensi
London; Cassell LTD., 1978
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Cadena, Mariano Velazquez de la, author
Follett, 1962
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The World Bank, 1996
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