An ellipse has $OB$ as semi minor axis, $F$ and $F$' its focii and theangle $FBF$' is a right angle. Then the eccentricity of the ellipse is :

Question

${1 \over {\sqrt 2 }}$

${1 \over 2}$

${1 \over 4}$

${1 \over {\sqrt 3 }}$

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