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

A. aggregation congestion occurring at the core layer where the different distribution layer devices feed traffic to the core-layer switches
B. speed mismatches when moving from a low-speed LAN environment to a higher speed WAN environment
C. flapping Ethernet interfaces caused by misconfigured LAN interfaces
D. flapping serial interfaces caused by misconfigured WAN interfaces
E. speed mismatches when moving from a high-speed LAN environment to a lower speed WAN environment