[单选题]
A stone, of mass m, is attached to a 6+1df( t zuutastrong string and whirle)cwmu+no ,yq7d in a vertical circle of radius r. At the exact bottom of the p)wcm7n q, +yuoath the tension in the string is 3 times the stone's weight. The stone's speed at this point is given by
A. $2\sqrt{gr}$
B. $\sqrt{2gr}$
C. $\sqrt{gr}$
D. $2gr$