Não informado. (1960). Educators Guide to Free Science Materials. Educators Progress Service.
Citace podle Chicago (17th ed.)Não informado. Educators Guide to Free Science Materials. Randolph, Wis: Educators Progress Service, 1960.
Citace podle MLA (8th ed.)Não informado. Educators Guide to Free Science Materials. Educators Progress Service, 1960.
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