$A$ Carnot engine $(E)$ is working between two temperatures $473 \ K$ and $273 \ K$. In a new system,two engines are used: engine $E_1$ works between $473 \ K$ and $373 \ K$,and engine $E_2$ works between $373 \ K$ and $273 \ K$. If $\eta_{12}$,$\eta_1$,and $\eta_2$ are the efficiencies of the engines $E$,$E_1$,and $E_2$ respectively,then:

• A
  $\eta_{12} < \eta_1 + \eta_2$
• B
  $\eta_{12} = \eta_1 \eta_2$
• C
  $\eta_{12} = \eta_1 + \eta_2$
• D
  $\eta_{12} \geq \eta_1 + \eta_2$