Two Carnot engines working in series

1. A.
   $\frac{2}{3}{\text{ }T}_{1} + \frac{3}{2}{\text{ }T}_{3}$
2. B.
   $\frac{1}{3}{\text{ }T}_{1} + \frac{2}{3}{\text{ }T}_{3}$
3. C.
   $\frac{3}{2}{\text{ }T}_{1} + \frac{1}{3}{\text{ }T}_{3}$
4. D.
   $\frac{2}{3}{\text{ }T}_{1} + \frac{1}{3}{\text{ }T}_{3}$

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1. A.
   same as the $1^{\text{st~}}$ case
2. B.
   always greater than the $1^{\text{st~}}$ case
3. C.
   always less than the $1^{\text{st~}}$ case
4. D.
   may increase or decrease with respect to the $1^{\text{st~}}$ case

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1. A.
   $\eta_{12} < \eta_{1} + \eta_{2}$
2. B.
   $\eta_{12} = \eta_{1}\eta_{2}$
3. C.
   $\eta_{12} = \eta_{1} + \eta_{2}$
4. D.
   $\eta_{12} \geq \eta_{1} + \eta_{2}$

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