In which situation would the trust boundary be located at the access layer?

A.
if the endpoints, both IP phones and PCs, are incapable of marking traffic properly

B.
if PCs are switched through an IP phone and the IP phone traffic can be trusted to mark both
traffic streams properly

C.
if the access layer switch cannot trust or re-mark incoming traffic from endpoints properly

D.
if there are endpoints that cannot be trusted and connect directly to the distribution layer