答案：B
$P\text{(第二題正確 $|$ 最少一題正確)}$
$P\text{(第二題正確 $|$ 最少一題正確)}$
$\begin{array}{cl}
= & \dfrac{P\text{(第二題正確及最少一題正確)}}{P\text{(最少一題正確)}} \\
= & \dfrac{\frac{3}{4} \times \frac{1}{3} + \frac{1}{4} \times \frac{1}{3}}{\frac{1}{4} \times \frac{2}{3} + \frac{3}{4} \times \frac{1}{3} + \frac{1}{4} \times \frac{1}{3}} \\
= & \dfrac{2}{3}
\end{array}$
