[hugoroy]

Here's how it goes:

You develop X and distribute it to B.

B modifies X (becomes X.1) and because B has been well advised by lawyers, B knows that modification of software is an act restricted under copyright law, and so B went to go read the LICENSE file and found Section 13 of AGPL. As a result because he/she is diligent, B ensures that the source code of X.1 can be accessed by putting a link to a server in X.1's user interface.

See Section 13 of AGPL:

B's "modified version must prominently offer all users interacting with it remotely through a computer network (if your version supports such interaction) an opportunity to receive the Corresponding Source of your version by providing access to the Corresponding Source from a network server at no charge ..."

B distributes X.1 to C

C runs X.1 which is a version that already offers all users a way to get the source code.