The value of 'a' for which the function \(f(x)= \rm \left \{\begin{matrix}\rm \frac{1-\cos4x}{x^2}, & \rm x<0\\\ \rm a, & \rm x=0\\\ \rm \frac{\sqrt x}{\sqrt{16+\sqrt x}-4}, & \rm x>0\end{matrix}\right.\) is continuous at x = 0, is
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2
8
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4
\(\frac{1}{2}\)