Hierarchical Network Design is now considered to be the best practice industry$-$wide to design networks that are reliable, resilient, scalable, and also cost$-$effective. Initially, networks were designed in a Flat Topology where the end devices were connected using Hubs and Switches. a$.$ Explain why the need of the hierarchical network design over the flat topology design method. b$.$ State and explain three layers used in the hierarchical design method. c$.$ Give three advantages of hierarchical design.