$\displaystyle \lim _ {n \rightarrow \infty} {\frac{2} {n} \sum _ {k=1} ^ {n} \sqrt {4- \left (\frac{2k} {n} \right) ^ {2}}}$ 의 값은?

• ①$\pi$
• ②$\displaystyle \frac{5} {4} \pi$
• ③$\displaystyle \frac{3} {2} \pi$
• ④$\displaystyle \frac{7} {4} \pi$
• ⑤$2 \pi$