Emelia and Lola have created a new software application that allows messaging site
subscribers to analyze the message statistics in order to more easily assess and interact
with followers. They have decided to license this software according to the GNU GPL

Version 3. What does this license allow them to do with their software?

A.
The software enters the public domain for free use, but any changes made in the future
can be kept secret, allowing them to profitably sell the modified software.

B.
They can only sell this software through a not-for-profit company.

C.
They can sell this software, but only if they make the software code available on a public
server.

D.
The software enters the public domain for free use and can be altered by other
programmers, who must then make their changes freely available as well.