The angle between the pair of straight lines $y^2 \cos^2 \theta - xy \cos^2 \theta + x^2 (\sin^2 \theta - 1)=0$, is :
( A ) $\frac{2 \pi}{3}$
( B ) $\frac{\pi}{3}$
( C ) $\frac{\pi}{4}$
( D ) $\frac{\pi}{2}$