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 lowest position and has a speed `u` . Find the magnitude of the change in its velocity as it reaches a position, where the string is horizontal.
A. `sqrt((u^(2)-2gl))`
B. `sqrt(2gl)`
C. `sqrt((u^(2)-gl))`
D. `sqrt((2u^(2)-gl))`
