Which of the following elements is the most critical to online security according to the author

A．In science, it is not difficult to distinguish fact from fiction.
B．Today few people have any notions of the meaning of science.
C．Practical-minded people can understand the meaning and objectives of pure science.
D．Scientists engaged in theoretical research should not be blamed for ignoring the practical side of their discoveries.

A．some OS suppliers are not trustworthy
B．it still fails to shield users from intrusions
C．antivirus software is not updated in due time
D．some terminals are not installed with such a system