单选题 An advanced persistent threat (APT) is a covert (     ) attack on a computer network where the attacker gains and maintains (     ) access to the targeted network and remains undetected for a significant period. An APT isa sophisticated, long-term and (     )attack, usually orchestrated by nation-state groups, cor well-organized criminal enterprises. During the time between infection and remediation the hacker will often monitor, intercept, and relay information and sensitive data. The intention of an APT is usually to (回答此处 ) or steal data rather than cause a network outage, denial of service or infect systems with malware. APTs often use social engineering tactics or exploit security ( ) in networks, applications or files to plant malware on target systems. A successful advanced persistent threat can be extremely effective and beneficial to the attacker.