答案：A
$\begin{array}{rcl}
x & = & kt^a \\
\log_3 x & = & \log_3 (kt^a) \\
\log_3 x & = & a\log_3 t +\log_3 k
\end{array}$
$\begin{array}{rcl}
x & = & kt^a \\
\log_3 x & = & \log_3 (kt^a) \\
\log_3 x & = & a\log_3 t +\log_3 k
\end{array}$
留意圖像的 $y$ 截距為 $-4$。所以，可得
$\begin{array}{rcl}
\log_3 k & = & -4 \\
k & = & 3^{-4} \\
& = & \dfrac{1}{81}
\end{array}$
