Engineering Thermodynamics
A closed 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 specific heat at constant volume is

The amount of heat required to raise the temperature of unit mass of gas through one degree, at constant pressure
Any one of the above
The amount of heat required to raise the temperature of unit mass of gas through one degree, at constant volume
The amount of heat required to raise the temperature of 1 kg of water through one degree

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