___39___ is used to ensure the confidentiality, integrity and authenticity of the two end points in the private network. ___40___, an application-layer protocol, authenticates each peer in an IPSec transaction. IKE negotiates security policy, determining which algorithm may be used to set up the tunnel. It also handles the exchange of session keys used for that one transaction.

Networks that use ___41___ to secure data traffic can automatically authenticate devices by using by using ___42___, which verify the identities of the two users who are sending information back and forth. IPSec can be ideal way to secure data in large networks that require secure connections among many devices.

Users deploying IPSec can ___43___ their network infrastructure without affecting the applications on individual computer. The protocol suite is available as a software-only upgrade to the network infrastructure. This alows security to be implemented without costly changes to each computer. Most important, IPSec allows interoperability among different network devices, PCs and other computing systems.

39（）

A. Certificate

B. Encryption

C. Tunnel

D. Presentation

中国对朝核问题的底线是（）

A.不管什么情况下，半岛都不能有核，无论是北方还是南方，无论是自己制造，还是引进部署。

B.不能用武力解决问题，那将使半岛生战、生乱，中国不会允许。

C.必要的时候将进一步强化对朝鲜的制裁

D.中国自身的正当国家安全利益必须得到有效维护和保障。