All the points in the set $S = \left\{ {{{\alpha + i} \over {\alpha - i}}:\alpha \in R} \right\}(i = \sqrt { - 1} )$ lie on a :

Question

All the points in the set
$S = \left\{ {{{\alpha + i} \over {\alpha - i}}:\alpha \in R} \right\}(i = \sqrt { - 1} )$ lie on a :

straight line whose slope is –1

straight line whose slope is 1.

circle whose radius is 1.

circle whose radius is $\sqrt 2$ .

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