If $A$ denotes the area of free surface of a liquid and $\bar{h}$ the depth of an orifice of area of cross-section $a$, below the liquid surface, then the velocity $v$ of flow through the orifice is given by
(A) $v=\sqrt{(2 g h)}$
(B) $v=\sqrt{(2 g h)} \sqrt{\left(\frac{A^{2}}{A^{2}-a^{2}}\right)}$
(C) $v=\sqrt{2 g h} \sqrt{\left(\frac{A}{A-a}\right)}$
(D) $v=\sqrt{2 g h} \sqrt{\left(\frac{A^{2}-a^{2}}{A^{2}}\right)}$