If $^{\prime} \theta$ ' is the angle between the asymptotes of the hyperbola $\frac{\mathrm{x}^{2}}{\mathrm{a}^{2}}-\frac{\mathrm{y}^{2}}{\mathrm{~b}^{2}}=1$, then $\cos \frac{\theta}{2}$ is
(a) $\sqrt{\mathrm{e}}$
(b) $\frac{1}{1+\mathrm{e}}$
(c) $\frac{1}{\sqrt{e}}$
(d) $\frac{1}{\mathrm{e}}$