Ans: B
The required probability
$\begin{array}{cl}
= & \dfrac{3}{7} \times \dfrac{2}{7} +\dfrac{3}{7}\times \dfrac{3}{7} \times \dfrac{3}{7}\times \dfrac{2}{7} +\dfrac{3}{7}\times \dfrac{3}{7}\times \dfrac{3}{7}\times \dfrac{3}{7} \times \dfrac{3}{7}\times \dfrac{2}{7} +\ldots \\
= &\dfrac{6}{49} \div \left(1 -\dfrac{9}{49}\right) \\
= & \dfrac{3}{20}
\end{array}$
