Consider a two-stage Carnot engine. In the first stage, heat $Q_1$ is absorbed at temperature $T$ and heat $Q_2$ is expelled at temperature $\alpha T$ (where $\alpha < 1$). In the second stage, heat $Q_2$ is absorbed at temperature $\alpha T$ and heat $Q_3$ is expelled at temperature $\beta T$ (where $\beta < \alpha$). The efficiency of the Carnot engine will be

• A
  $1-\alpha-\beta$
• B
  $1-\alpha$
• C
  $1-\beta$
• D
  $1-\alpha \beta$