An open system is one in which

• A. a) heat & work cross the boundary of the system, but the mass of the working substance does not
• B. b) mass of working substance crosses the boundary of the system but the heat & work do not
• C. c) both the heat & work as well as mass of the working substance cross the boundary of the system
• D. d) neither the heat & work nor the mass of the working substances cross the boundary of the system