Prices for Treasuries rose on Thursday as investors bought safe-haven U.S. debt after the head of the European Central Bank noted the risks facing the eurozone economy, cooling investors' fervor for riskier assets. Treasuries rose as stocks and the euro fell after ECB chief Mario Draghi said risks to the eurozone economy were to the downside and that economic weakness in the eurozone will likely prevail in coming months. "We've had this 'risk on'