The author mentions all of the following as reasons why plankton could be considered a human food source EXCEPT that it is ________.

A．to investigate the security of Internet sites
B．to improve the security of the Internet system
C．to prevent hackers from breaking into websites
D．to download useful programs and information

A．we should make full use of the Internet before security measures are strengthened
B．we should alert the most influential businessmen to the importance of security
C．influential businessmen should give priority to the improvement of Net security
D．net inhabitants should not let security measures affect their joy of surfing the Internet