水下浇注混凝土，如桩截面上存在夹泥层，会造成（）现象。 A.缩颈桩 B.断桩 C.吊

___57___ is used to ensure the confidentiality, integrity and authenticity of the two end points in the private network. ___58___, 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 ___59___ to secure data traffic can automatically authenticate devices by using by using ___60___, 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 ___61___ 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.

58（）

A. IPSec    

B. SSL

C. L2TP

D. The Internet Key Exchange

阅读下面的文字， 完成后面的题目。    

　　他焦急万分，左顾右盼，努力地想从不同的表情中看出点原因来，但大家全都默不作声。    

　　突然，他厉声呼唤我，当我走近他的身旁时，他把他的笔记本递给我，示意我写。我写下了下面这句话：“我恳求您别再继续了，回去以后我将向您说明理由。”他猛地一下跳到下面，冲我嚷叫道：“咱们快走！”他一口气跑回家，进得门来，他瘫软地跌坐在沙发上，双手掩面，他就这样一直待到吃饭。饭桌上，没法让他说一句话，一副痛苦不堪、颓丧无力的样子。  （有删改）

这段文字中“他”指的是____________。“恳求您别再继续”的“理由”是_______________。

X射线管阳极罩的作用是（）。

A.吸收二次电子

B.减少管壁电荷

C.提高工作稳定性

D.以上都对

流行病学研究方法使用最多的是

A．观察法

B．实验法

C．理论法

D．方法学研究

E．以上都不是

VLAN具有控制广播数据、增加网络安全性等功能。（）