We use the invariant interval to find the time $\Delta t$ it takes the pion to travel $30 \mbox{ m}$ in the lab frame:

$\begin{align*}\left(30 \mbox{ m}\right)^2 - c^2 \left(\Delta t\right)^2 = - c^2 \left(10^{-8} \mbox{ s}\right)^2 \Rightarrow...$
Therefore, the pion's speed in the laboratory frame is
$\begin{align*}\frac{30 \mbox{ m}}{\displaystyle \sqrt{\frac{\left(30 \mbox{ m} \right)^2 + \left(c \cdot 10^{-8} \mbox{ s} \r...$
Therefore, answer (D) is correct.

Solution to 2001 Problem 33