United States. Department of Labor
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Buku Referensi
Karch, Amy M.
J.B. Lippincott, 1989
Buku Teks
Karb, Virginia Burke
Mosby , 1996
Buku Teks
Turner, Paul
Macmillan, 1993
Buku Referensi
Ehrenpreis, Seymour
McGraw-Hill, 2001
Buku Referensi