12. There are two positive integers, $A$ and $B$. The sum of the digits of $A$ is $19$, and the sum of the digits of $B$ is 20. When the two numbers are added, there are two carries. Then, the sum of the digits of $(\mathbf{A}+\mathrm{B})$ is ( ).
【Analysis】Carrying over 1 time, the sum of digits decreases by 9, so the sum of digits of $\mathrm{A}+\mathrm{B}$ is $19+20-2 \times 9=21$.