Let the time taken by B alone to fill the tank be x hours

Time taken by A to fill the tank is $\mathrm{x}+\frac{\left(\mathrm{x}\right)\times 80}{100}=\frac{9\mathrm{x}}{5}\mathrm{hours}$

According to the formula, time taken by both the filling pipes to fill the tank is $\frac{\mathrm{xy}}{\mathrm{x}+\mathrm{y}}$

$$\begin{gathered} \Rightarrow 45=\frac{\left(\mathrm{x}\right)\times {\displaystyle \frac{9\mathrm{x}}{5}}}{\left(x\right)+{\displaystyle \frac{9x}{5}}} \\ \Rightarrow 45+\frac{14\mathrm{x}}{5}=\frac{9{\mathrm{x}}^2}{5} \\ \Rightarrow \mathrm{x}=\frac{45\times 14}{9}=70 \mathrm{hours} \end{gathered}$$

Hence option D is correct.