Ans: D
Note that the slope of the straight line $x+3y-211=0$ is $\dfrac{-1}{3}$,
Note that the slope of the straight line $x+3y-211=0$ is $\dfrac{-1}{3}$,
and the slope of the straight line $kx-3y+211=0$ is $\dfrac{k}{3}$.
Since the two lines are perpendicular to each other, then we have
$\begin{array}{rcl}
\dfrac{-1}{3} \times \dfrac{k}{3} & = & -1 \\
k & = & 9
\end{array}$
