2. A box contains $m$ red balls, 10 white balls, and $n$ black balls. Each ball is identical except for its color. If a ball is randomly drawn from the box, the probability of drawing a white ball is the same as the probability of not drawing a white ball. What is the relationship between $m$ and $n$? ( ( ).
(A) $m+n=10$
(B) $m+n=5$
(C) $m=n=10$
(D) $m=2, n=3$
2. A.

The total number of balls in the box is $m+n+10$, the probability of drawing a white ball is $P=\frac{10}{m+n+10}$; the probability of not drawing a white ball is $P^{\prime}=\frac{m+n}{m+n+10}$.
According to the problem, $\frac{10}{m+n+10}=\frac{m+n}{m+n+10}$. Therefore, $m+n=10$.