A function f(x) is defined as f(x) = \(\left\{\begin{array}{c} \frac{1-\cos 4 x}{x^{2}} ; x<0 \\ a ; x=0 \\ \frac{\sqrt{x}}{\sqrt{(16+\sqrt{x})}-4} ; x>0 \end{array}\right.\)
If the function f(x) is continuous at x = 0, then the value of a is:
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4
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6
3
8
4
10