A heat engine changes thermal energy into mechanical work. It receives thermal energy $Q_{\text{in}}$ from a heat source and transfers thermal energy $Q_{\text{out}}$ to a heat sink, so that useful work $W$ is produced. State the efficiency of this engine?
- A$\frac{W}{Q_{\text{in}} + Q_{\text{out}}}$
- B$\frac{W}{Q_{\text{in}} - Q_{\text{out}}}$
- C$\frac{W}{Q_{\text{in}}}$
- D$\frac{W}{Q_{\text{out}}}$