The argument of $\large\frac{1-\sqrt 3i}{1+\sqrt 3i}=?$

Question

(A) $\large\frac{\pi}{3}$

(B) $\large\frac{2\pi}{3}$

(C) $\large\frac{4\pi}{3}$

(D) $\large-\frac{\pi}{3}$