A Cisco router has been configured, and the copy running-config startup-config command has been issued. When the router is power cycled, the router prompts with: "Would you like to enter the initial configuration dialog? [yes/no]" Why has this occurred?（）

A. There is an error in the router DRA
B. Te IOS image is corrupt.
C. The configuration register is set to 0x2142.
D. The TFTP server that contains the router configuration file is unreachable.
E. A boot system configuration command has placed the router into setup mode.