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 (     ) 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 (回答此处 ) 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.