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(2(u^(2)-gL))`
