Why is it necessary to map L2 QoS marking to L3 QoS marking?

A.
because L2 QoS marking may not cross the network end to end

B.
because L2 QoS markings such as CoS are only local to the router or switch

C.
because L2 QoS marking can only identify up to 8 traffic classes

D.
because routers do not support L2 QoS marjings

E.
because L2 Qos marking cannot be trusted