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 $\frac{\mathrm{xy}}{\mathrm{x}-\mathrm{y}}$ 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 pipes are opened simultaneously.

Hence option C is correct.