극한 $\displaystyle \lim _ {x \rightarrow 0} \left \{\frac{1} {x ^ {2}} - \frac{1} {x \left (e ^ {x} - 1 \right)} - \frac{1} {2 \left (e ^ {x} - 1 \right)} \right \}$ 의 값은?

• ①$\displaystyle \frac{1} {6}$
• ②$\displaystyle \frac{1} {3}$
• ③$\displaystyle \frac{1} {2}$
• ④$\displaystyle \frac{2} {3}$