* Corn was underpinned by slow farmer selling, firm cash markets and good demand. * Hopes for a resolution to the euro zone debt crisis weakened the dollar, lifted grains and crude oil and gave a boost to equity markets. * Showers slowed harvest in portions of the eastern US Midwest but the bulk of the Midwest remained conducive to combining. Frost seen killing weeds and boosting harvest progress, causing no harm to mature crops. * US cash corn basis bids were firm, supported by slow farmer selling and good demand. * Key support for December at its 200-day moving average of $6.49 and key resistance at the 100-day MA of $6.78-1/2.