12. In the coordinate plane, the line $l: 3 x-5 y+40$ $=0$ is rotated counterclockwise by $45^{\circ}$ around the point $(20,20)$ to obtain the line $l^{\prime}$. Then the x-intercept of the line $l^{\prime}$ is ( ).
(A) 10
(B) 15
(C) 20
(D) 25
(E) 30
12. B.

Given that the slope of line $l$ is $\frac{3}{5}$, after rotating line $l$ counterclockwise by $45^{\circ}$, the slope of line $l^{\prime}$ is
$$
\frac{\frac{3}{5}+1}{1-\frac{3}{5} \times 1}=4
$$

Thus, the equation of line $l^{\prime}$ is $y-20=4(x-20)$.
Let $y=0$, then $x=15$.