What is the most common cause of network congestion？（）

A、A.aggregation congestion occurring at the core layer where the different distribution layer devices feed traffic to the core-layer switches

B、B.speed mismatches when moving from a low-speed LAN environment to a higher speed WAN environment

C、C.flapping Ethernet interfaces caused by misconfigured LAN interfaces

D、D.flapping serial interfaces caused by misconfigured WAN interfaces

E、E.speed mismatches when moving from a high-speed LAN environment to a lower speed WAN environment