A venturimeter having a throat diameter of $0.1$ m is used to estimate the flow rate of a horizontal pipe having a diameter of $0.2$ m. For an observed pressure difference of $2$ m of water head and coefficient of discharge equal to unity, assuming that the energy losses are negligible, the flow rate (in $m^3/s$) through the pipe is approximately equal to

1. $0.500$
2. $0.150$
3. $0.050$
4. $0.015$