I don't know how to do the bottom part

This exercise contains only parts b and c. Click the icon to view information about the steppers. The product structure is as follows: b) Choose the correct time-phased structure. c) Develop a net material requirements plan for F (enter your responses as whole numbers). \begin{tabular}{cccc} \hline Item & Lead Time & On-Hand Inventory & Components \\ \hline Stepper & 2 & 20 & A(2),B(3),C(1) \\ A & 1 & 20 & D(2),F(3) \\ B & 2 & 20 & E(1),F(2) \\ C & 2 & 10 & D(1),E(3) \\ D & 1 & 15 & \\ E & 2 & 5 & \\ F & 2 & 20 & \\ \hline \end{tabular}