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http://www.envplan.com/abstract.cgi?id=a211381

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http://www.envplan.com/abstract.cgi?id=a211427


3.ENVIRONMENTAL QUALITY AND LAND PRODUCTIVITY: A COMPARISON OF THE AGRICULTURAL LAND BASE OF THE USSR AND NORTH AMERICA
N. C. FIELD
Canadian Geographer / Le Géographe canadien
Volume 12, Issue 1 , Pages1 - 14
1968 Canadian Association of Geographers
http://www3.interscience.wiley.com/journal/119866367/abstract?CRETRY=1&SRETRY=0

把前几天发了求助没人应答的再发一遍。请高人相助,xiexie