곡선 $\displaystyle r (t) = < \ln \left (\sec t \right), \cos t, \sin t > \left (0 \leq t \leq \frac{\pi} {3} \right)$ 의 길이는?

• ①$\ln \left (1+ \sqrt {3} \right)$
• ②$\ln \left (2+ \sqrt 3 \right)$
• ③$2 \ln \left (1+ \sqrt 3 \right)$
• ④$2 \ln \left (2+ \sqrt 3 \right)$