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

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

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

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