From a high tower, at time t = 0, one stone is dropped from rest and simultaneously another stone is projected vertically up with an initial velocity. The graph of distanceS between the two stones plotted against time t will be

At time t, let the displacement of first stone be S1=12gt2 and that of the second stone beS2=ut−12gt2Distance between two stones at time t:S=S1+S2=u ⇒ S=utSo the graph should be a straight line passing through origin as shown in option (a).