Engineering Thermodynamics
In order that a cycle be reversible, following must be satisfied

When beat is being rejected, temperature of cold source and working substance should be same
When heat is being absorbed, temperature of hot source and working substance should be same
All of these
Free expansion or friction resisted expansion/compression process should not be encountered

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Engineering Thermodynamics
The specific heat at constant volume is

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

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