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) √u2-2gL
(b) √2gL
(c) √u2-gl
(d) √2(u2-gL)