A block of mass \[m\] at the end of a string is whirled round in a vertical circle of radius \[R\]. The critical speed of the block at the top of its swing below which the string would slacken before the block reaches the top is [DCE 1999, 2001]

• A.             \[Rg\]  
• B.             \[{{(Rg)}^{2}}\]
• C.             \[R/g\] 
• D.             \[\sqrt{Rg}\]