A cyclist starts from the point \( P \) of a circular ground of radius \( 2 \, \text{km} \) and travels along its circumference to the point \( S \). The displacement of a cyclist is:
\( \sqrt{8} \, \text{km} \)
\( 6 \, \text{km} \)
\( 8 \, \text{km} \)
\( 4 \, \text{km} \)