Choose all the following statements that are correct. a. A reversible heat engine is more efficient that an irreversible heat engine when the two engines are operated between the same hot and cold reservoirs. b. A heat engine can be made more efficient by raising the temperature of the hot reservoir. c. Any reversible heat engine is more efficient than any irreversible engine. d. The Kelvin Statement of the Second Law of Thermodynamics says that the efficiency of a heat engine must be less than 1