2. On a plane under the sun, there is a ball. At a certain moment, the farthest point of the ball's shadow from the point where the ball touches the plane is $10 \mathrm{~m}$. At the same moment, the shadow of a meter stick perpendicular to the plane is $2 \mathrm{~m}$ long. What is the radius of the ball (unit: $\mathrm{m}$, assuming the sunlight is parallel, and the meter stick is a line segment)?
A. $\frac{5}{2}$
B. $9-4 \sqrt{5}$
C. $8 \sqrt{10}-23$
D. $10 \sqrt{5}-20$
2. D As shown in the figure, it is easy to see that $A B=5, A D=5 \sqrt{5}$. From $\triangle C E A \backsim \triangle D B A, \frac{r}{5-r}=\frac{10}{5 \sqrt{5}}$, $r=10 \sqrt{5}-20$.