In a client$-$server paradigm

Select one:

a$.$ the server must be running all the time but the client needs to be running only when it is needed.

b$.$ the client must be running all the time but the server needs to be running only when it is needed

c$.$ both the server and the client need to be running only when they are needed.

d$.$ both the server and the client must be running all the time.