3. A water pool is equipped with 9 inlet and outlet pipes numbered $1,2, \cdots, 9$. Some are only for inlet, and some are only for outlet. It is known that the time required to fill the pool when certain pipes are opened is as follows. If all 9 pipes are opened at the same time, it will take ( ) hours to fill the pool.
A. 1
B. 2
C. 3
D. 4
3. Let it take $t$ hours, then $\left(\frac{1}{2}+\frac{1}{4}+\cdots+\frac{1}{16}+\frac{1}{31}+\frac{1}{62}+\cdots+\frac{1}{496}\right) t=2$,

i.e., $\left[\frac{\frac{1}{2}\left(1-\frac{1}{16}\right)}{1-\frac{1}{2}}+\frac{\frac{1}{31}\left(1-\frac{1}{32}\right)}{1-\frac{1}{2}}\right] t=2$, solving this gives $t=2$, so 9 pipes working together need 2 hours. Therefore, the answer is $\mathrm{B}$.