What is the most common cause of network congestion？（）
- Aaggregation congestion occurring at the core layer where the different distribution layer devices feed traffic to the core-layer switches
- Bspeed mismatches when moving from a low-speed LAN environment to a higher speed WAN environment
- Cflapping Ethernet interfaces caused by misconfigured LAN interfaces
- Dflapping serial interfaces caused by misconfigured WAN interfaces
- Espeed mismatches when moving from a high-speed LAN environment to a lower speed WAN environment