# An ellipse has OB as semi minor axis, $F$ and $F'$ its foci and th angle $FBF'$ is a right angle. Then the eccentricity of the ellipse is :
( A ) $\frac{1}{\sqrt{2}}$
( B ) $\frac{1}{2}$
( C ) $\frac{1}{\sqrt{3}}$
( D ) $\frac{1}{4}$