Engineering Thermodynamics
An isolated system is one in which

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

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Engineering Thermodynamics
The mass of flue gas per kg of fuel is the ratio of the

Mass of oxygen in 1 kg of fuel to the mass of oxygen in 1 kg of flue gas
Mass of oxygen in 1 kg of flue gas to the mass of oxygen in 1 kg of fuel
Mass of carbon in 1 kg of flue gas to the mass of carbon in 1 kg of fuel
Mass of carbon in 1 kg of fuel to the mass of carbon in 1 kg of flue gas

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