11. For hyperbola $x ^ { 2 } / \left( \cos ^ { 2 } a \right) - y ^ { 2 } / \left( \sin ^ { 2 } a \right) = 1$ which of the following remains constant with change in ' a ' :\\
(a) abscissae of vertices\\
(b) abscissa of foci
\section*{III askllTians ||}
... Powered By IITians\\
(c) eccentricity\\
(d) directrix\\