What type of refrigerants are normally used in cascade CO2 refrigeration systems? Question 17 options: a) CO2 refrigerant in high-temperature systems and HFC refrigerant in low-temperature systems b) HFC refrigerant mixed with CO2 in all systems c) CO2 refrigerant in both the high- and low-temperature systems; HFC in medium-temperature systems d) HFC refrigerant in medium- to high-temperature systems and CO2 in low-temperature systems