In case of vertical circular motion of a particle by a thread of length r if the tension in the thread is zero at an angle $$30^{\circ}$$ shown in figure, the velocity at the bottom point (A) of the circular path is
(g = gravitational acceleration)

A

$$\sqrt{4gr}$$

B

$$\sqrt{5gr}$$

C

$$\sqrt{\frac{7}{2}gr}$$

D

$$\sqrt{\frac{5}{2}gr}$$