$\displaystyle \cos ^ {-1} \left (\cos \frac{4} {3} \pi \right)$ 의 값은?

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