The client$-$server paradigm. Which of the characteristics below are associated with a client$-$server approach to structuring network applications $($as opposed to a P$2$P approach$)?$ a$.$ There is a server that is always on$.$ b$.$ There is not a server that is always on$.$ c$.$ HTTP uses this application structure. d$.$ A process requests service from those it contacts and will provide service to processes that contact it$.$ e$.$ There is a server with a well$-$known server IP address.