7. In a water supply system, water flows in from pipes 1 and 2 and goes out from pipes 3 and 4 as shown. If all the pipes have the same diameter, which of the following must be correct? 2 a) the sum of the flow velocities in 1 and 2 is equal to that in 3 and 4 b) the sum of the flow velocities in 1 and 3 is equal to that in 2 and 4 c) the sum of the flow velocities in 1 and 4 is equal to that in 2 and 3 d) the flow velocities in 1 and 2 is equal to that in 3 and 4