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The prototype , which was an amphibian with a retractable wheeled undercarriage which swung forward into streamlined casings in the leading edges of the wing s , was powered by two Wright SGR-1820-F Cyclone nine-cylinder air-cooled radial engine s mounted above the wing , each driving a tractor propeller .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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