3. As shown in the figure, a semicircle $O$ is inscribed in the trapezoid $ABCD$ on the base $AB$, and is tangent to the other two sides $BC$, $CD$, $DA$. If $BC=2$, $DA=3$, then the length of $AB$ ( ).
(A) is 4
(B) is 5
(C) is 6
(D) cannot be determined
3. Connect $O C, O D$. Let the radius of the semicircle $O$ be $r$, then in $\triangle A O D$, the distances on sides $A O$ and $D A$ are both $r$, so $A O=D A$. Similarly, $B O=B C$. Therefore, $A B=B C+D A=5$.

