According to the formula, if pipe A can comletely fill a tank in x hours and pipe B can empty the same tank in y hours, then the time taken to fill the tank when both the pipes are opened simultaneously is $\frac{\mathrm{xy}}{\mathrm{y}-\mathrm{x}}$ hours.

so, $\frac{45\times 120}{120-45}=\frac{5400}{75}=72 \mathrm{minutes}$

The tank will be filled in 72 minutes when both the pipes are opened simultaneously.

Hence option D is correct