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