Oil prices retreated below $80 a barrel Tuesday in Asia as investors mulled whether sluggish U.S. crude demand justified a 14 percent rally over the last three weeks. Benchmark crude for April delivery was down 34 cents to $79.97 a barrel at midday Singapore time in electronic trading on the New York Mercantile Exchange. The contract added 25 cents to settle at $80.31 on Monday. Oil has jumped from $69.59 a barrel on Feb. 5 on investor optimism that the global economy will rebound strongly from recession last year. Yet growing inventories of crude, gasoline and diesel fuel suggest demand in the U.S. remains weak.