111- If $2^a = \displaystyle\lim_{x \to \frac{\pi}{4}} \dfrac{\sqrt{\cos x} - \sqrt{\sin x}}{\cos(x + \frac{\pi}{4})}$, then what is $a$? (1) $-\dfrac{1}{2}$ \hfill (2) $-\dfrac{1}{4}$ \hfill (3) $\dfrac{1}{4}$ \hfill (4) $\dfrac{1}{2}$