The passage is mainly about ______.

A．an invention will not benefit the inventor unless it is reduced to commerical practice
B．products are actually inventions which were made a long time ago
C．it is much cheaper to buy an old patent than a new one
D．patent experts often recommend. patents to others by conducing a search through dead patents

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