A new floor has been added to the enterprise campus network， providing client access for desktop PCs， laptops， and IP phones for employees and guests. During verification， a few desktop PCs have limited corporate access but not Internet access. Most other desktop PCs， all laptops， and IP phones are working correctly. Additional information indicates that the desktop PCs are obtaining IP host addresses from the DHCP server， which is intended for only the IP phones. Where should troubleshooting about this connectivity issue be focused？（）

A. DNS
B. first-hop redundancy protocol
C. DHCP server and options
D. VLAN mapping
E. client network configuration
F. routing protocol