\textbf{8.} For the function $f(x) = \sqrt{x - 2\sqrt{mx-1}}$ on its natural domain, the line $y = 12 - x$ intersects the graph at 10 points on the $x$-axis. What is the value of $f(m+4)$?
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(1) $\dfrac{1}{2}$ \hfill (2) $\dfrac{1}{4}$ \hfill (3) $2$ \hfill (4) $1$
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