Network security consists of policies and practices to prevent and monitor（ ）access,misuse,modification, or denial of a computer network and network-accessible resources.Network security involves the authorization of access to data in a network,which is controlled by the network（请作答此空）. Users choose or are assigned an ID and password or other authenticating information that allows them to access to information and programs within their authority.Network security secures the network,as well as protecting and overseeing operations being done. The most common and simple way of protecting a network resource is by assigning it a（ ）name and a corresponding password. Network security starts with authentication.Once authenticated a （ ） enforces policies such as what services are allowed to be accessed by the network users. Though effective to prevent unauthorized access, this component may fail to check potentially harmful content such as computer（ ）or Trojans being transmitted over the network
A.user
B.agent
C.server
D.administrator