A heat engine changes thermal energy into mechanical work. It absorbs thermal energy $Q_{\text{in}}$ from a heat source and releases thermal energy $Q_{\text{out}}$ to a heat sink, thereby producing useful work $W$. Calculate the efficiency of this engine.
- A$\dfrac{W}{Q_{\text{in}} + Q_{\text{out}}}$
- B$\dfrac{W}{Q_{\text{in}} - Q_{\text{out}}}$
- C$\dfrac{W}{Q_{\text{in}}}$
- D$\dfrac{W}{Q_{\text{out}}}$