(1) is used to ensure the confidentiality, integrity and authenticity of the two end points in the private network. (2) , 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 (3) to secure data traffic can automatically authenticate devices by using (4) , 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 (5) 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.

1（）

A．Certificate

B．Encryption

C．Tunnel

D．Presentation

叶绿体是植物进行光合作用的细胞器，下面有关叶绿体的叙述正确的是（）

A.在囊状结构的薄膜上分布有吸收光能的色素

B.叶绿体的外膜和内膜上分布有少量的色素

C.与光合作用有关的酶都分布在叶绿体基质中

D.CO2能穿过内外膜和类囊体膜进入类囊体腔中被利用