What is the main difference between Erlang$-B$ and Erlang$-C$ usages?

Select one:

Erlang$-$B: large systems. Erlang$-$C: small systems.

Erlang$-$B: non$-$queued systems $($blocking$).$ Erlang$-$C: queued systems.

Both are the same.

Erlang$-$B: queued systems. Erlang$-$C: non$-$queued systems $($blocking$).$