$1.$ a$)$ b$)$ Describe four main characteristics of an infrastructure. Use relevant examples from four different digital infrastructures to illustrate each characteristic. Why is the architecture of digital infrastructure almost always modular and layered? Answers might be built around core concepts such as: shared, heterogeneous, scaled, evolving, open, standardised, path dependent. Part b$)$ then asks for an account of why digital infrastructures are in general modular and layered. That does not require a long explanation of what layered and modular means. A good answer here will focus on the need to support change and evolution, to allow heterogeneity in both the use$/$users of an infrastructure and in the way the installed base is adapted $($e$.$g$.$ from the $$top$$ and from the bottom of the layers$).$