Which statement represents the second law of thermodynamics? A. Thermal energy always flows naturally from a cold object to a hot object and never naturally from a hot object to a cold object. B. There are some processes that can remove thermal energy from a source and convert it entirely into mechanical energy. C. Some mechanical processes can have an efficiency of 100% with no loss of thermal energy. D. Work is needed to make thermal energy move from a cold substance to hot substance.