By 2015, consumers in the U.S. likely will spend nearly $4 billion on daily-deal offers provided by companies such as Groupon and LivingSocial, says a report released Thursday morning. The report, by research firm BIA/Kelsey, offered a three-level estimate for the amount of spending on daily deals. It said $3.9 billion is the “most likely” scenario to play out by 2015. But under the most “optimistic” scenario, it says sales could reach $6 billion, while the firm’s “low-end projection” is $2.1 billion.