For the Hyperbola $\frac{x^2}{\cos^2 \alpha} - \frac{y^2}{\sin^2 \alpha} = 1$, which of the following remains constant when $\alpha$ varies?

Question

( A ) Directrix
( B ) Eccentricity
( C ) Abscissae of vertices
( D ) Abscissae of foci