Determine Reynolds number of the flow?

why are the velocities Vb and Vc used for all the losses. For example the loss calculated for the 90 degree elbow in the first part of the question is calculated using the final velocity would the velocity Va not be better suited or even the average velocity?

Heavy machine tool lubricating oil (ref. Appendix C applied fluid mechanics) is pumped through six rectangular slots in a heat exchanger (oil flows in the white rectangles in section view a-a) as shown below. You may assume that the oil flows through the heat exchanger with a constant temperature of 40°C. If the flow rate from A to B is 3 cm³/sec determine the Reynolds number for the oil flow. Is the flow laminar or turbulent?

please answer question ASAP