答案：B
$\begin{array}{rcl}
\dfrac{m}{m+20} & = & \dfrac{1}{m} \\
m^2 & = & m+20 \\
m^2 – m – 20 & = & 0 \\
(m-5)(m+4) & = & 0
\end{array}$
$\begin{array}{rcl}
\dfrac{m}{m+20} & = & \dfrac{1}{m} \\
m^2 & = & m+20 \\
m^2 – m – 20 & = & 0 \\
(m-5)(m+4) & = & 0
\end{array}$
所以，$m=5$ 或 $m=-4$。
由於黃球的數目必為正數，所以 $m=5$。
