Weerth, Georg, author
Aufbau, 1956
Buku Teks
Americana Corporation, 1980
Buku Referensi
Stimpson, George
Harper and Brothers Publisher, 1946
Buku Referensi
Weideman, Hugh
Thomas Y. Crowell Company, 1958
Buku Referensi
Merriam-Webster, 2000
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