根据《会计档案管理办法》的规定，会计档案保管期限分为永久和定期两类。定期保管的会计档案，其最长期限是( )。

A．10年
B．15年
C．25年
D．30年

Most computer systems are (16) to two different groups of attacks: Insider attacks and outsider attacks. A system that is known to be (17) to an outsider attack by preventing.
(18) from outside can still be vulnerable to the insider attacks accomplished by abusive usage of (19) users. Detecting such abusive usage as well as attacks by outsides not only provides information on damage assessment, but also helps to prevent future attacks. These attacks are usually (20) by tools referred to as Intrusion Detection Systems.

A．vulnerable

B．week

C．sensitively

D．helpless