Backman, Jules
The Ronald Press Company, 1953
Buku Teks
Backman, Jules
The Brookings Institution, 1943
Buku Teks
Hamilton, Walton
McGraw-Hill Book Company , 1938
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Nourse, Edwin G
The Brookings Institution, 1938
Buku Teks
George Allen and Unwin , 1951
Buku Teks