A rigid bar of mass $m$ and length $L$ is released from rest in the horizontal position. What is the rod's angular velocity when it has rotated through $90^{\circ}$?

A. $(\frac{g}{3L})^{1/2}$
B. $(\frac{g}{L})^{1/2}$
C. $(\frac{12g}{L})^{1/2}$
D. $(\frac{3g}{L})^{1/2}$