Emelia and Lola Have Created a New Software Application That

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) They can only sell this software through a not-for-profit company.
B) They can sell this software, but only if they make the software code available on a public server.
C) 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.
D) 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.