The efficiency of a Carnot engine is $\eta$ when its hot and cold reservoirs are maintained at temperatures $T_1$ and $T_2$, respectively. To increase the efficiency to $1.5 \eta$, the increase in temperature $(\Delta T)$ of the hot reservoir, while keeping the cold reservoir constant at $T_2$, is

• A
  $\frac{T_1 T_2}{(1-\eta)(1-1.5 \eta)}$
• B
  $\frac{0.5 T_2 \eta}{(1-1.5 \eta)(1-\eta)}$
• C
  $\frac{T_1}{1-\eta}-\frac{T_2}{1-1.5 \eta}$
• D
  $\frac{(1-\eta)(1-1.5 \eta)}{T_1 T_2}$