The cost of living in the United States held steady for the second consecutive month in December as energy prices eased, the Labor Department said Thursday. The consumer price index showed zero change, while the core CPI, which excludes food and energy, slipped slightly to 0.1 percent, the department reported. For the full-year 2011, CPI rose 3.0 percent, the largest December-to-December increase since 2007. In 2010 CPI rose 1.5 percent.