According to the formula, If Two pipes A and B can fill the tank in x and y min respectively. If both pipes are opened together, then the time after which pipe B should be closed so that the tank is full in t min is $\left\{ \mathrm{y} \left[1-\frac{\mathrm{t}}{\mathrm{x}}\right] \right\}$ min.

So, $\left\{ 16 \left[1-\frac{9}{12}\right] \right\}=16\times \frac{3}{12}=4 \min$

The tank will be filled in 4 min both the pipes are opened simultaneously.

Hence option A is correct