The word "underscore" in the last sentence of the text probably means"______".

A．hostile online attacks are still rampant
B．fighting online attacks is a never-ending battle
C．other elements than technology are at work for greater security
D．new operating systems should be updated constantly to fight viruses

A．IT culture.
B．Operating systems.
C．Desktop antivirus software.
D．The efforts by online security vendors.