If $a = \lim_{n \to \infty} \sum_{k=1}^{n} \frac{2n}{n^2+k^2}$ and $f(x) = \sqrt{\frac{1-\cos x}{1+\cos x}}$,$x \in (0, 1)$,then:

  • A
    $2 \sqrt{2} f \left(\frac{a}{2}\right) = f'\left(\frac{a}{2}\right)$
  • B
    $f \left(\frac{a}{2}\right) f'\left(\frac{a}{2}\right) = \sqrt{2}$
  • C
    $\sqrt{2} f \left(\frac{a}{2}\right) = f'\left(\frac{a}{2}\right)$
  • D
    $f \left(\frac{a}{2}\right) = \sqrt{2} f'\left(\frac{a}{2}\right)$

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