Why is flash memory erased prior to upgrading the IOS image from the TFTP server?

A.
in order for the router to use the new image as the default, it must be the only IOS image in flash.

B.
Flash memory on Cisco routers can contain only a single IOS image.

C.
Erasing current flash content is requested during the copy dialog.

D.
The router cannot verify that the Cisco IOS image currently in flash is valid.

Explanation:
We can keep multiple IOS files on flash memory if there is enough space. When you try to copy the IOS to flash memory, it will ask you to erase current contents of flash memory. If there is enough free space to copy IOS you can type no to erase the contents of flash. If there is not enough space the router will require that the current file is erased first.