Sumner, W. G., & Keller, A. G. (1914). The challenge of facts, and other essays. Yale University Press.
शिकागो शैली (17वां संस्करण) प्रशस्ति पत्रSumner, William Graham, और Albert Galloway Keller. The Challenge of Facts, and Other Essays. New Haven: Yale University Press, 1914.
एमएलए (9वां संस्करण) प्रशस्ति पत्रSumner, William Graham, और Albert Galloway Keller. The Challenge of Facts, and Other Essays. Yale University Press, 1914.
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