What is the difference between the backbone model and the wireframe model of protein visualization?

Select one:

a$.$ Backbone model shows all of the atoms in the protein and wireframe model does not show all of the atoms in the protein

b$.$ Backbone model does not include $R$ groups and the wireframe model includes $R$ groups

c$.$ Backbone model includes $R$ groups and the wireframe model does not include $R$ groups

d$.$ Backbone model shows the general shape AND R groups