Copper and oil firm up, gold falls from 4-wk high

* CRB sees biggest monthly loss in a year * Euronext wheat futures top performer in May * U.S. silver weakest commodity this month By Marcy Nicholson NEW YORK, May 31 (Reuters) - Copper, oil and soft commodities rose on Tuesday, with the red metal tapping a four-week peak on growing hopes for a second aid package for Greece and as the U.S. dollar dropped, prompting investors to seek riskier

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