$A$ large open tank containing water has two holes in its wall. $A$ square hole of side $a$ is made at a depth $y$ and a circular hole of radius $r$ is made at a depth $16y$ from the surface of the water. If an equal amount of water comes out through both the holes per second,then the relation between $r$ and $a$ will be:

• A
  $r=\frac{2 a}{\pi}$
• B
  $r=\frac{a}{2 \sqrt{\pi}}$
• C
  $r=\frac{a}{2 \pi}$
• D
  $r=\frac{2 a}{\sqrt{\pi}}$