A liquid having area of free surface ' $A^{\prime}$ has an orifice at a depth $^{\prime} h^{\prime}$ with an area $a^{\prime}$, below the liquid surface, then the velocity $v$ of flow through the orifice is :
(a) $v=\sqrt{2 g h}$
(b) $v=\sqrt{2 g h} \sqrt{\frac{A^{2}}{A^{2}-a^{2}}}$
(c) $v=\sqrt{2 g h} \sqrt{\frac{A}{A-a}}$
(d) $v=\sqrt{2 g h} \sqrt{\frac{1^{2}-a^{2}}{A^{2}}}$