答案:C
依題意略繪下圖。
![](https://solvingmaster.space/wp-content/uploads/2023/07/2022PaperII23_fig01-300x280.png)
由於 $PQRS$ 為一菱形,則 $PQ = QR$、$ST = QT$ 及 $\angle QTR =90^\circ$。由此,可得
$\begin{array}{rcl}
\dfrac{PQ}{ST} & = & \dfrac{QR}{QT} \\
\dfrac{PQ}{ST} & = & \dfrac{1}{\sin \theta}
\end{array}$