3. $C$
$S_{\triangle A B O_{1}}=\frac{1}{2}\left|O_{1} A\right| \cdot\left|O_{1} B\right| \sin \angle A O_{1} B=\frac{9}{2} \sin \angle A O_{1} B \leqslant \frac{9}{2}$ It is easy to see that the equality can be achieved.
3. Given the line $l_{1}: 2 x+5 y=1$ and the circle $C: x^{2}+y^{2}-2 x+4 y=4$ with center $O_{1}$, a moving line $l_{2}$ is parallel to $l_{1}$ and intersects the circle $C$ at points $A, B$. Then the maximum value of $S_{\triangle A B B_{1}}$ is $(\mathrm{)}$.
A. 2
B. 6
C. $\frac{9}{2}$
D. $\frac{25}{2}$