\textbf{137-} Point $S(2,1)$ is the vertex of a parabola whose axis of symmetry is parallel to the $x$-axis, and it passes through point $(5,0)$. What is the equation of its directrix?
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(1) $y = \dfrac{1}{4}$ \hfill (2) $y = \dfrac{1}{2}$ \hfill (3) $y = \dfrac{3}{4}$ \hfill (4) $y = \dfrac{3}{2}$
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