A stone is tied to a string of length 'l' and is whirled in a vertical circle with the other end of the string as 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 velocity as it reaches a position where the string is horizontal (g being acceleration due to gravity) is :-

Question

( A ) ${\sqrt{ 2(u^{2}-gl})}$
( B ) $U-\sqrt{U^{2}-2gl}$
( C ) $\sqrt{U^{2}-gl}$
( D ) $\sqrt{2gl}$