Engineering Thermodynamics
An open system is one in which

Both energy and mass cross the boundaries of the system
Mass does not cross boundaries of the system, though energy may do so
Mass crosses the boundary but not the energy
Neither mass nor energy crosses the boundaries of the system

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Engineering Thermodynamics
The condition for the reversibility of a cycle is

All of these
All the processes, taking place in the cycle of operation, must be extremely slow
The pressure and temperature of the working substance must not differ, appreciably, from those of the surroundings at any stage in the process
The working parts of the engine must be friction free

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