Water is coming out from a tap and falls vertically downwards. At the tap opening, the stream diameter is $20$ $mm$ with uniform velocity of $2$ $m/s$. Acceleration due to gravity is $9.81$ $m/s^2$. Assuming steady, inviscid flow, constant atmospheric pressure everywhere and neglecting curvature and surface tension effects, the diameter in mm of the stream $0.5$ $m$ below the tap is approximately
- $10$
- $15$
- $20$
- $25$