5. The graph of the function $y=\sqrt{3} \cos x-\sin x$ is shifted to the left by $m$ units, and the resulting graph is symmetric about the $y$-axis. Then the smallest positive value of $m$ is ( ).
A. $\frac{\pi}{6}$
B. $\frac{\pi}{3}$
C. $\frac{2 \pi}{3}$
D. $\frac{5 \pi}{6}$