A stone tied to a string of length \(L\) is whirled in a vertical circle with the other end of the string at the centre. At a certain instant of time, the stone is at its lowest position and has a speed \(u\). The magnitude of the change in its velocity as it reaches a position where the string is horizontal is
(a) \(\sqrt{u^{2}-2 g L}\)
(b) \(\sqrt{2 g L}\)
(c) \(\sqrt{u^{2}-g} \mid\)
(d) \(\sqrt{2}\left(u^{2}-g L\right)\)