21. Given a sphere $O$ with radius 6 that is tangent to all sides of a triangle with side lengths 15, 15, and 24, the distance from point $O$ to the plane of the triangle is ( ).
(A) $2 \sqrt{3}$
(B) 4
(C) $3 \sqrt{2}$
(D) $2 \sqrt{5}$
(E) 5
$$
r=\frac{s}{p}=\frac{\frac{1}{2} \times 24 \times 9}{\frac{1}{2}(15+15+24)}=4
$$

Then $d=\left|O O^{\prime}\right|=\sqrt{6^{2}-4^{2}}=2 \sqrt{5}$.