According to the above given formula, if pipe A can comletely fill a tank in x hours and pipe B can comletely fill a tank in y hours, then the time taken by both pipes together to fill the tank is $\frac{\mathrm{xy}}{\mathrm{x}-\mathrm{y}}$ hours.

So, $\frac{8\times 16}{16-8}=\frac{128}{8}=16 \mathrm{hours}$

Therefore, the tank will be filled in 16 hours when both the pipe are opened simultaneously.

Hence option C is correct.