Q. An open system is one in which

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