Leffler, G. L. (1949). Average prices of gas, water and electric light and power companies in Pennsylvania since 1939.
शिकागो शैली (17वां संस्करण) प्रशस्ति पत्रLeffler, George Leland. Average Prices of Gas, Water and Electric Light and Power Companies in Pennsylvania Since 1939. State College, Pa, 1949.
एमएलए (8वां संस्करण) प्रशस्ति पत्रLeffler, George Leland. Average Prices of Gas, Water and Electric Light and Power Companies in Pennsylvania Since 1939. 1949.
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