Gregg, Pauline
George G. Harrap, 1952
Buku Teks
A.G. Champion...[et al.]
Baltimore, Md., USA: Arnold, 1987
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Lenin, Vladimir I.
Foreign Languages Publishing House , [date of publication not identified]
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Allen, G.C.
Longmans, Green and Co., 1961
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Walker, Gilbert
Heimann , 1957
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