Why is QoS best applied in the aggregation layer of the data center network?

A.
It is the only layer that uses VLANs.

B.
QoS should be applied to the core layer, which is strictly Layer 3.

C.
Layer 2 statistics collection from the aggregation layer is easier than from the core layer.

D.
It is usually where Layer 2 and Layer 3 are demarcated.

Explanation: