The value of \( \displaystyle \lim_{n \to \infty} \dfrac{1}{n} \displaystyle \sum_{r=0}^{2n-1} \dfrac{n^{2}}{n^{2}+4 r^{2}} \) is:
\( \dfrac{1}{2}\tan^{-1}(2) \)
\( \tan^{-1}(4) \)
\( \dfrac{1}{4}\tan^{-1}(4) \)
\( \dfrac{1}{2}\tan^{-1}(4) \)