U.S. inventories at the wholesale level post a month-on-month gain in December 2011, a positive sign for the gradually-improving economy, the U.S. Department of Commerce\'s figures revealed Thursday.Inventories at the wholesale level rose 1 percent in December from the revised November level to 473.2 billion U.S. dollars, 10 percent higher than a year earlier.Sales at the wholesale level gained 1.3 percent in December from revised November level to 413.1 billion dollars, or 11.8 percent up from a year ago.The December inventories/sales ratio stood at 1.15, the same with prior month, but slightly lower than the figure of 1.16 in December 2010.The economy grew at an annual pace of 2.8 percent in the fourth quarter of 2011, mainly driven by an upturn in business inventory. But it is not certain whether the momentum could last in the coming quarters.