nly one of the following statements is true. Which is it? (a) Mechanical energy can never be completely converted into thermal energy. (b) Thermal energy can never be completely converted into mechanical energy. (c) All Carnot heat engines have the same efficiency, no matter what reservoirs they use. (d) It is impossible to transfer heat from a cold reservoir to a hot reservoir. (e) Irreversible processes violate the second law of thermodynamics