Two Carnot heat engines are connected in series such that the sink of the first engine is heat source of the second. Efficiencies of the engines are $$\eta_1$$ and $$\eta_2$$ respectively. Net efficiency $$\eta$$ of the combination is given by

A

$$\eta=\eta_1+\eta_2$$

B

$$\eta=\frac{\eta_1\eta_2}{\eta_1+\eta_2}$$

C

$$\eta=\eta_1+\eta_2-\eta_1\eta_2$$

D

$$\eta=\eta_1-\eta_2(1-\eta_1)$$