答案：B
所獲代幣的數目的期望值
所獲代幣的數目的期望值
$\begin{array}{cl}
= & \dfrac{1}{10} \times 90 + \dfrac{3}{10} \times 20 + \dfrac{6}{10} \times 10 \\
= & 21
\end{array}$
