甲、乙两人用两个掷骰子做游戏，将两个骰子同时抛出，如果出现两个5点，那么甲赢；如

一平行板电容器充电后与电源断开如图所示，负极板接地，在两极板间有一正电荷（电量很小）固定在P点，如图所示，以E表示两极板间的场强，U表示电容器的电压，W表示正电荷在P点的电势能。若保持负极板不动，将正极板移到图中虚线所示的位置，则

A．U变小，E不变

B．E变大，W变大

C．U变小，W不变

D．U不变，W不变

正常成人的血量相当于体重的（）

A.7%～8%

B.10%～15%

C.20%～30%

D.40%～50%

E.50%～60%

计算二重积分

其中D=(x，y)|y≥0，1≤x²+y²≤2x。

___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

典型离心泵向水塔供水，随塔内水位升高，流量会( )

A．加大
B．减小
C．不变
D．不能确定