The passage is mainly about ______.

A．A patent holder must publicize the details of his invention when its legal period is over.
B．When a patent becomes out of effect, it can be re-patented or extended if necessary.
C．It is necessary for an inventor to apply for a patent before he makes his invention public.
D．One can get all the details of a patented invention from a library attached to the patent office.

A．make public
B．give reward to
C．steal and use
D．take and change