What problems could arise during maintenance if a large software system were designed in such a way that all of its data elements were global? In an object-oriented program, what does declaring an...

What problems could arise during maintenance if a large software system were designed in such a way that all of its data elements were global? In an object-oriented program, what does declaring an instance variable to be public or private indicate about data coupling? What would be the rationale behind a preference toward declaring instance variables as private?