Não informado. (1969). Science activities. Science Activities Publishing.
Παραπομπή σε μορφή Chicago (17η εκδ.)Não informado. Science Activities. Washington: Science Activities Publishing, 1969.
Παραπομπή σε μορφή MLA (8th εκδ.)Não informado. Science Activities. Science Activities Publishing, 1969.
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