Correct Answer - A
At time `t`, let the desplacement of first stone be `S_(1) =(1)/(2)g t^(2)` and that of the second stone be `S_(2) =ut -0(1)/(2) g t^(2)`
Distance between two stones at time `t`:
`S=S_(2)+S_(2)=u rArr S=ut` ltnbrgt
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So the graph should be a straight line passing through origin as shown in option `a`.